From 653c7c772dc3ee85848f01204f6315efc9af139a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tom=C3=A1=C5=A1=20Votruba?= <info@tomasvotruba.cz>
Date: Tue, 14 Nov 2017 21:21:23 +0100
Subject: [PATCH] Upgrade to PHP 7.1 (#150)

* upgrade to PHP 7.1

* bump travis and composer to PHP 7.1

* fix tests
---
 .travis.yml                                   |  5 +--
 composer.json                                 |  2 +-
 src/Phpml/Association/Apriori.php             | 12 +++---
 src/Phpml/Classification/DecisionTree.php     |  8 ++--
 .../Classification/Ensemble/AdaBoost.php      |  8 ++--
 src/Phpml/Classification/Ensemble/Bagging.php |  4 +-
 .../Classification/KNearestNeighbors.php      |  2 +-
 src/Phpml/Classification/Linear/Adaline.php   |  4 +-
 .../Classification/Linear/DecisionStump.php   | 14 +++----
 .../Linear/LogisticRegression.php             | 12 +++---
 .../Classification/Linear/Perceptron.php      | 10 +++--
 src/Phpml/Classification/MLPClassifier.php    |  2 +-
 src/Phpml/Classification/NaiveBayes.php       | 10 ++---
 src/Phpml/Classification/SVC.php              |  2 +-
 .../Classification/WeightedClassifier.php     |  2 +-
 src/Phpml/Clustering/DBSCAN.php               |  2 +-
 src/Phpml/Clustering/FuzzyCMeans.php          |  8 ++--
 src/Phpml/Clustering/KMeans.php               |  4 +-
 src/Phpml/Clustering/KMeans/Cluster.php       |  8 ++--
 src/Phpml/Clustering/KMeans/Point.php         |  4 +-
 src/Phpml/Clustering/KMeans/Space.php         |  6 +--
 src/Phpml/CrossValidation/RandomSplit.php     |  2 +-
 src/Phpml/CrossValidation/Split.php           |  4 +-
 .../CrossValidation/StratifiedRandomSplit.php |  2 +-
 src/Phpml/Dataset/FilesDataset.php            |  4 +-
 .../EigenTransformerBase.php                  |  2 +-
 src/Phpml/DimensionReduction/KernelPCA.php    | 10 ++---
 src/Phpml/DimensionReduction/LDA.php          |  2 +-
 src/Phpml/DimensionReduction/PCA.php          |  4 +-
 .../FeatureExtraction/TfIdfTransformer.php    |  8 ++--
 .../TokenCountVectorizer.php                  | 18 ++++-----
 src/Phpml/Helper/OneVsRest.php                | 10 ++---
 .../Helper/Optimizer/ConjugateGradient.php    |  4 +-
 src/Phpml/Helper/Optimizer/GD.php             |  8 ++--
 src/Phpml/Helper/Optimizer/StochasticGD.php   |  4 +-
 src/Phpml/Helper/Trainable.php                |  2 +-
 .../LinearAlgebra/EigenvalueDecomposition.php | 10 ++---
 src/Phpml/Math/Statistic/Covariance.php       |  6 +--
 src/Phpml/Math/Statistic/Mean.php             |  2 +-
 src/Phpml/Metric/ClassificationReport.php     |  4 +-
 src/Phpml/Metric/ConfusionMatrix.php          |  2 +-
 src/Phpml/ModelManager.php                    |  2 +-
 src/Phpml/NeuralNetwork/Layer.php             |  6 +--
 .../NeuralNetwork/Network/LayeredNetwork.php  |  4 +-
 .../Network/MultilayerPerceptron.php          | 24 ++++++------
 src/Phpml/NeuralNetwork/Node/Input.php        |  2 +-
 src/Phpml/NeuralNetwork/Node/Neuron.php       |  6 +--
 .../NeuralNetwork/Node/Neuron/Synapse.php     |  4 +-
 .../Training/Backpropagation.php              |  2 +-
 src/Phpml/Pipeline.php                        |  8 ++--
 src/Phpml/Preprocessing/Imputer.php           | 10 ++---
 src/Phpml/Preprocessing/Normalizer.php        | 16 ++++----
 src/Phpml/Regression/LeastSquares.php         |  4 +-
 src/Phpml/Regression/SVR.php                  |  2 +-
 src/Phpml/SupportVectorMachine/Kernel.php     |  8 ++--
 .../SupportVectorMachine.php                  | 12 +++---
 src/Phpml/SupportVectorMachine/Type.php       | 10 ++---
 tests/Phpml/Association/AprioriTest.php       | 30 +++++++--------
 .../DecisionTree/DecisionTreeLeafTest.php     |  2 +-
 .../Phpml/Classification/DecisionTreeTest.php | 14 +++----
 .../Classification/Ensemble/AdaBoostTest.php  |  2 +-
 .../Classification/Ensemble/BaggingTest.php   | 14 +++----
 .../Ensemble/RandomForestTest.php             |  2 +-
 .../Classification/KNearestNeighborsTest.php  |  8 ++--
 .../Classification/Linear/AdalineTest.php     |  4 +-
 .../Linear/DecisionStumpTest.php              |  2 +-
 .../Classification/Linear/PerceptronTest.php  |  4 +-
 .../Classification/MLPClassifierTest.php      | 22 +++++------
 tests/Phpml/Classification/NaiveBayesTest.php |  6 +--
 tests/Phpml/Classification/SVCTest.php        |  6 +--
 tests/Phpml/Clustering/DBSCANTest.php         |  4 +-
 tests/Phpml/Clustering/FuzzyCMeansTest.php    |  2 +-
 tests/Phpml/Clustering/KMeansTest.php         |  6 +--
 .../Phpml/CrossValidation/RandomSplitTest.php | 12 +++---
 .../StratifiedRandomSplitTest.php             |  4 +-
 tests/Phpml/Dataset/ArrayDatasetTest.php      |  4 +-
 tests/Phpml/Dataset/CsvDatasetTest.php        |  8 ++--
 tests/Phpml/Dataset/Demo/GlassDatasetTest.php |  2 +-
 tests/Phpml/Dataset/Demo/IrisDatasetTest.php  |  2 +-
 tests/Phpml/Dataset/Demo/WineDatasetTest.php  |  2 +-
 tests/Phpml/Dataset/FilesDatasetTest.php      |  4 +-
 .../DimensionReduction/KernelPCATest.php      |  4 +-
 tests/Phpml/DimensionReduction/LDATest.php    |  4 +-
 tests/Phpml/DimensionReduction/PCATest.php    |  6 +--
 .../Phpml/FeatureExtraction/StopWordsTest.php | 10 ++---
 .../TfIdfTransformerTest.php                  |  2 +-
 .../TokenCountVectorizerTest.php              |  6 +--
 tests/Phpml/Math/ComparisonTest.php           |  4 +-
 tests/Phpml/Math/Distance/ChebyshevTest.php   | 10 ++---
 tests/Phpml/Math/Distance/EuclideanTest.php   | 10 ++---
 tests/Phpml/Math/Distance/ManhattanTest.php   | 10 ++---
 tests/Phpml/Math/Distance/MinkowskiTest.php   | 12 +++---
 tests/Phpml/Math/Kernel/RBFTest.php           |  2 +-
 .../LinearAlgebra/EigenDecompositionTest.php  |  2 +-
 tests/Phpml/Math/MatrixTest.php               | 38 +++++++++----------
 tests/Phpml/Math/ProductTest.php              |  2 +-
 tests/Phpml/Math/SetTest.php                  | 20 +++++-----
 .../Phpml/Math/Statistic/CorrelationTest.php  |  4 +-
 tests/Phpml/Math/Statistic/CovarianceTest.php |  2 +-
 tests/Phpml/Math/Statistic/GaussianTest.php   |  2 +-
 tests/Phpml/Math/Statistic/MeanTest.php       | 14 +++----
 .../Math/Statistic/StandardDeviationTest.php  |  6 +--
 tests/Phpml/Metric/AccuracyTest.php           |  8 ++--
 .../Phpml/Metric/ClassificationReportTest.php | 10 ++---
 tests/Phpml/Metric/ConfusionMatrixTest.php    |  6 +--
 tests/Phpml/ModelManagerTest.php              |  4 +-
 .../ActivationFunction/BinaryStepTest.php     |  2 +-
 .../ActivationFunction/GaussianTest.php       |  2 +-
 .../HyperboliTangentTest.php                  |  2 +-
 .../ActivationFunction/PReLUTest.php          |  2 +-
 .../ActivationFunction/SigmoidTest.php        |  2 +-
 .../ThresholdedReLUTest.php                   |  2 +-
 tests/Phpml/NeuralNetwork/LayerTest.php       | 10 ++---
 .../Network/LayeredNetworkTest.php            |  6 +--
 tests/Phpml/NeuralNetwork/Node/BiasTest.php   |  2 +-
 tests/Phpml/NeuralNetwork/Node/InputTest.php  |  4 +-
 .../NeuralNetwork/Node/Neuron/SynapseTest.php |  4 +-
 tests/Phpml/NeuralNetwork/Node/NeuronTest.php |  8 ++--
 tests/Phpml/PipelineTest.php                  |  8 ++--
 tests/Phpml/Preprocessing/ImputerTest.php     | 14 +++----
 tests/Phpml/Preprocessing/NormalizerTest.php  | 12 +++---
 tests/Phpml/Regression/LeastSquaresTest.php   |  8 ++--
 tests/Phpml/Regression/SVRTest.php            |  6 +--
 .../DataTransformerTest.php                   |  4 +-
 .../SupportVectorMachineTest.php              | 12 +++---
 .../Tokenization/WhitespaceTokenizerTest.php  |  4 +-
 .../Phpml/Tokenization/WordTokenizerTest.php  |  4 +-
 127 files changed, 419 insertions(+), 420 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index 7c5b9bb..48162fd 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,9 +4,6 @@ matrix:
   fast_finish: true
 
   include:
-    - os: linux
-      php: '7.0'
-
     - os: linux
       php: '7.1'
 
@@ -18,7 +15,7 @@ matrix:
       language: generic
       env:
         - _OSX=10.11
-        - _PHP: php70
+        - _PHP: php71
 
 before_install:
   - if [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then /usr/bin/env bash tools/prepare_osx_env.sh ; fi
diff --git a/composer.json b/composer.json
index 18ae994..8db4934 100644
--- a/composer.json
+++ b/composer.json
@@ -17,7 +17,7 @@
         }
     },
     "require": {
-        "php": ">=7.0.0"
+        "php": "^7.1"
     },
     "require-dev": {
         "phpunit/phpunit": "^6.0",
diff --git a/src/Phpml/Association/Apriori.php b/src/Phpml/Association/Apriori.php
index ee9c383..f1398d2 100644
--- a/src/Phpml/Association/Apriori.php
+++ b/src/Phpml/Association/Apriori.php
@@ -11,13 +11,13 @@ class Apriori implements Associator
 {
     use Trainable, Predictable;
 
-    const ARRAY_KEY_ANTECEDENT = 'antecedent';
+    public const ARRAY_KEY_ANTECEDENT = 'antecedent';
 
-    const ARRAY_KEY_CONFIDENCE = 'confidence';
+    public const ARRAY_KEY_CONFIDENCE = 'confidence';
 
-    const ARRAY_KEY_CONSEQUENT = 'consequent';
+    public const ARRAY_KEY_CONSEQUENT = 'consequent';
 
-    const ARRAY_KEY_SUPPORT = 'support';
+    public const ARRAY_KEY_SUPPORT = 'support';
 
     /**
      * Minimum relative probability of frequent transactions.
@@ -116,7 +116,7 @@ class Apriori implements Associator
     /**
      * Generate rules for each k-length frequent item set.
      */
-    private function generateAllRules()
+    private function generateAllRules(): void
     {
         for ($k = 2; !empty($this->large[$k]); ++$k) {
             foreach ($this->large[$k] as $frequent) {
@@ -130,7 +130,7 @@ class Apriori implements Associator
      *
      * @param mixed[] $frequent
      */
-    private function generateRules(array $frequent)
+    private function generateRules(array $frequent): void
     {
         foreach ($this->antecedents($frequent) as $antecedent) {
             if ($this->confidence <= ($confidence = $this->confidence($frequent, $antecedent))) {
diff --git a/src/Phpml/Classification/DecisionTree.php b/src/Phpml/Classification/DecisionTree.php
index f99b032..653b1bf 100644
--- a/src/Phpml/Classification/DecisionTree.php
+++ b/src/Phpml/Classification/DecisionTree.php
@@ -14,8 +14,8 @@ class DecisionTree implements Classifier
 {
     use Trainable, Predictable;
 
-    const CONTINUOUS = 1;
-    const NOMINAL = 2;
+    public const CONTINUOUS = 1;
+    public const NOMINAL = 2;
 
     /**
      * @var array
@@ -72,7 +72,7 @@ class DecisionTree implements Classifier
         $this->maxDepth = $maxDepth;
     }
 
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         $this->samples = array_merge($this->samples, $samples);
         $this->targets = array_merge($this->targets, $targets);
@@ -354,7 +354,7 @@ class DecisionTree implements Classifier
     /**
      * Used to set predefined features to consider while deciding which column to use for a split
      */
-    protected function setSelectedFeatures(array $selectedFeatures)
+    protected function setSelectedFeatures(array $selectedFeatures): void
     {
         $this->selectedFeatures = $selectedFeatures;
     }
diff --git a/src/Phpml/Classification/Ensemble/AdaBoost.php b/src/Phpml/Classification/Ensemble/AdaBoost.php
index 9fdd65b..5bdca1b 100644
--- a/src/Phpml/Classification/Ensemble/AdaBoost.php
+++ b/src/Phpml/Classification/Ensemble/AdaBoost.php
@@ -84,7 +84,7 @@ class AdaBoost implements Classifier
     /**
      * Sets the base classifier that will be used for boosting (default = DecisionStump)
      */
-    public function setBaseClassifier(string $baseClassifier = DecisionStump::class, array $classifierOptions = [])
+    public function setBaseClassifier(string $baseClassifier = DecisionStump::class, array $classifierOptions = []): void
     {
         $this->baseClassifier = $baseClassifier;
         $this->classifierOptions = $classifierOptions;
@@ -93,7 +93,7 @@ class AdaBoost implements Classifier
     /**
      * @throws \Exception
      */
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         // Initialize usual variables
         $this->labels = array_keys(array_count_values($targets));
@@ -149,7 +149,7 @@ class AdaBoost implements Classifier
             $classifier->setSampleWeights($this->weights);
             $classifier->train($this->samples, $this->targets);
         } else {
-            list($samples, $targets) = $this->resample();
+            [$samples, $targets] = $this->resample();
             $classifier->train($samples, $targets);
         }
 
@@ -216,7 +216,7 @@ class AdaBoost implements Classifier
     /**
      * Updates the sample weights
      */
-    protected function updateWeights(Classifier $classifier, float $alpha)
+    protected function updateWeights(Classifier $classifier, float $alpha): void
     {
         $sumOfWeights = array_sum($this->weights);
         $weightsT1 = [];
diff --git a/src/Phpml/Classification/Ensemble/Bagging.php b/src/Phpml/Classification/Ensemble/Bagging.php
index ebc7528..8d2bbf9 100644
--- a/src/Phpml/Classification/Ensemble/Bagging.php
+++ b/src/Phpml/Classification/Ensemble/Bagging.php
@@ -106,7 +106,7 @@ class Bagging implements Classifier
         return $this;
     }
 
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         $this->samples = array_merge($this->samples, $samples);
         $this->targets = array_merge($this->targets, $targets);
@@ -117,7 +117,7 @@ class Bagging implements Classifier
         $this->classifiers = $this->initClassifiers();
         $index = 0;
         foreach ($this->classifiers as $classifier) {
-            list($samples, $targets) = $this->getRandomSubset($index);
+            [$samples, $targets] = $this->getRandomSubset($index);
             $classifier->train($samples, $targets);
             ++$index;
         }
diff --git a/src/Phpml/Classification/KNearestNeighbors.php b/src/Phpml/Classification/KNearestNeighbors.php
index c7783d8..a261631 100644
--- a/src/Phpml/Classification/KNearestNeighbors.php
+++ b/src/Phpml/Classification/KNearestNeighbors.php
@@ -26,7 +26,7 @@ class KNearestNeighbors implements Classifier
     /**
      * @param Distance|null $distanceMetric (if null then Euclidean distance as default)
      */
-    public function __construct(int $k = 3, Distance $distanceMetric = null)
+    public function __construct(int $k = 3, ?Distance $distanceMetric = null)
     {
         if (null === $distanceMetric) {
             $distanceMetric = new Euclidean();
diff --git a/src/Phpml/Classification/Linear/Adaline.php b/src/Phpml/Classification/Linear/Adaline.php
index 2a649ad..64d25a4 100644
--- a/src/Phpml/Classification/Linear/Adaline.php
+++ b/src/Phpml/Classification/Linear/Adaline.php
@@ -9,12 +9,12 @@ class Adaline extends Perceptron
     /**
      * Batch training is the default Adaline training algorithm
      */
-    const BATCH_TRAINING = 1;
+    public const BATCH_TRAINING = 1;
 
     /**
      * Online training: Stochastic gradient descent learning
      */
-    const ONLINE_TRAINING = 2;
+    public const ONLINE_TRAINING = 2;
 
     /**
      * Training type may be either 'Batch' or 'Online' learning
diff --git a/src/Phpml/Classification/Linear/DecisionStump.php b/src/Phpml/Classification/Linear/DecisionStump.php
index 014dceb..e1486a6 100644
--- a/src/Phpml/Classification/Linear/DecisionStump.php
+++ b/src/Phpml/Classification/Linear/DecisionStump.php
@@ -14,7 +14,7 @@ class DecisionStump extends WeightedClassifier
 {
     use Predictable, OneVsRest;
 
-    const AUTO_SELECT = -1;
+    public const AUTO_SELECT = -1;
 
     /**
      * @var int
@@ -86,7 +86,7 @@ class DecisionStump extends WeightedClassifier
     /**
      * @throws \Exception
      */
-    protected function trainBinary(array $samples, array $targets, array $labels)
+    protected function trainBinary(array $samples, array $targets, array $labels): void
     {
         $this->binaryLabels = $labels;
         $this->featureCount = count($samples[0]);
@@ -146,7 +146,7 @@ class DecisionStump extends WeightedClassifier
      * points to be probed. The more split counts, the better performance but
      * worse processing time (Default value is 10.0)
      */
-    public function setNumericalSplitCount(float $count)
+    public function setNumericalSplitCount(float $count): void
     {
         $this->numSplitCount = $count;
     }
@@ -171,7 +171,7 @@ class DecisionStump extends WeightedClassifier
             // Before trying all possible split points, let's first try
             // the average value for the cut point
             $threshold = array_sum($values) / (float) count($values);
-            list($errorRate, $prob) = $this->calculateErrorRate($targets, $threshold, $operator, $values);
+            [$errorRate, $prob] = $this->calculateErrorRate($targets, $threshold, $operator, $values);
             if ($split == null || $errorRate < $split['trainingErrorRate']) {
                 $split = ['value' => $threshold, 'operator' => $operator,
                         'prob' => $prob, 'column' => $col,
@@ -181,7 +181,7 @@ class DecisionStump extends WeightedClassifier
             // Try other possible points one by one
             for ($step = $minValue; $step <= $maxValue; $step += $stepSize) {
                 $threshold = (float) $step;
-                list($errorRate, $prob) = $this->calculateErrorRate($targets, $threshold, $operator, $values);
+                [$errorRate, $prob] = $this->calculateErrorRate($targets, $threshold, $operator, $values);
                 if ($errorRate < $split['trainingErrorRate']) {
                     $split = ['value' => $threshold, 'operator' => $operator,
                         'prob' => $prob, 'column' => $col,
@@ -203,7 +203,7 @@ class DecisionStump extends WeightedClassifier
 
         foreach (['=', '!='] as $operator) {
             foreach ($distinctVals as $val) {
-                list($errorRate, $prob) = $this->calculateErrorRate($targets, $val, $operator, $values);
+                [$errorRate, $prob] = $this->calculateErrorRate($targets, $val, $operator, $values);
 
                 if ($split == null || $split['trainingErrorRate'] < $errorRate) {
                     $split = ['value' => $val, 'operator' => $operator,
@@ -289,7 +289,7 @@ class DecisionStump extends WeightedClassifier
         return $this->binaryLabels[1];
     }
 
-    protected function resetBinary()
+    protected function resetBinary(): void
     {
     }
 
diff --git a/src/Phpml/Classification/Linear/LogisticRegression.php b/src/Phpml/Classification/Linear/LogisticRegression.php
index bfbeb44..e8881be 100644
--- a/src/Phpml/Classification/Linear/LogisticRegression.php
+++ b/src/Phpml/Classification/Linear/LogisticRegression.php
@@ -11,17 +11,17 @@ class LogisticRegression extends Adaline
     /**
      * Batch training: Gradient descent algorithm (default)
      */
-    const BATCH_TRAINING = 1;
+    public const BATCH_TRAINING = 1;
 
     /**
      * Online training: Stochastic gradient descent learning
      */
-    const ONLINE_TRAINING = 2;
+    public const ONLINE_TRAINING = 2;
 
     /**
      * Conjugate Batch: Conjugate Gradient algorithm
      */
-    const CONJUGATE_GRAD_TRAINING = 3;
+    public const CONJUGATE_GRAD_TRAINING = 3;
 
     /**
      * Cost function to optimize: 'log' and 'sse' are supported <br>
@@ -97,7 +97,7 @@ class LogisticRegression extends Adaline
      * Sets the learning rate if gradient descent algorithm is
      * selected for training
      */
-    public function setLearningRate(float $learningRate)
+    public function setLearningRate(float $learningRate): void
     {
         $this->learningRate = $learningRate;
     }
@@ -106,7 +106,7 @@ class LogisticRegression extends Adaline
      * Lambda (λ) parameter of regularization term. If 0 is given,
      * then the regularization term is cancelled
      */
-    public function setLambda(float $lambda)
+    public function setLambda(float $lambda): void
     {
         $this->lambda = $lambda;
     }
@@ -139,7 +139,7 @@ class LogisticRegression extends Adaline
     /**
      * Executes Conjugate Gradient method to optimize the weights of the LogReg model
      */
-    protected function runConjugateGradient(array $samples, array $targets, \Closure $gradientFunc)
+    protected function runConjugateGradient(array $samples, array $targets, \Closure $gradientFunc): void
     {
         if (empty($this->optimizer)) {
             $this->optimizer = (new ConjugateGradient($this->featureCount))
diff --git a/src/Phpml/Classification/Linear/Perceptron.php b/src/Phpml/Classification/Linear/Perceptron.php
index d5f424b..77eb717 100644
--- a/src/Phpml/Classification/Linear/Perceptron.php
+++ b/src/Phpml/Classification/Linear/Perceptron.php
@@ -88,12 +88,12 @@ class Perceptron implements Classifier, IncrementalEstimator
         $this->maxIterations = $maxIterations;
     }
 
-    public function partialTrain(array $samples, array $targets, array $labels = [])
+    public function partialTrain(array $samples, array $targets, array $labels = []): void
     {
         $this->trainByLabel($samples, $targets, $labels);
     }
 
-    public function trainBinary(array $samples, array $targets, array $labels)
+    public function trainBinary(array $samples, array $targets, array $labels): void
     {
         if ($this->normalizer) {
             $this->normalizer->transform($samples);
@@ -111,7 +111,7 @@ class Perceptron implements Classifier, IncrementalEstimator
         $this->runTraining($samples, $targets);
     }
 
-    protected function resetBinary()
+    protected function resetBinary(): void
     {
         $this->labels = [];
         $this->optimizer = null;
@@ -148,6 +148,8 @@ class Perceptron implements Classifier, IncrementalEstimator
     /**
      * Trains the perceptron model with Stochastic Gradient Descent optimization
      * to get the correct set of weights
+     *
+     * @return void|mixed
      */
     protected function runTraining(array $samples, array $targets)
     {
@@ -169,7 +171,7 @@ class Perceptron implements Classifier, IncrementalEstimator
      * Executes a Gradient Descent algorithm for
      * the given cost function
      */
-    protected function runGradientDescent(array $samples, array $targets, \Closure $gradientFunc, bool $isBatch = false)
+    protected function runGradientDescent(array $samples, array $targets, \Closure $gradientFunc, bool $isBatch = false): void
     {
         $class = $isBatch ? GD::class : StochasticGD::class;
 
diff --git a/src/Phpml/Classification/MLPClassifier.php b/src/Phpml/Classification/MLPClassifier.php
index 64e6f50..b76091d 100644
--- a/src/Phpml/Classification/MLPClassifier.php
+++ b/src/Phpml/Classification/MLPClassifier.php
@@ -45,7 +45,7 @@ class MLPClassifier extends MultilayerPerceptron implements Classifier
     /**
      * @param mixed $target
      */
-    protected function trainSample(array $sample, $target)
+    protected function trainSample(array $sample, $target): void
     {
 
         // Feed-forward.
diff --git a/src/Phpml/Classification/NaiveBayes.php b/src/Phpml/Classification/NaiveBayes.php
index 08073da..97f734a 100644
--- a/src/Phpml/Classification/NaiveBayes.php
+++ b/src/Phpml/Classification/NaiveBayes.php
@@ -13,9 +13,9 @@ class NaiveBayes implements Classifier
 {
     use Trainable, Predictable;
 
-    const CONTINUOS = 1;
-    const NOMINAL = 2;
-    const EPSILON = 1e-10;
+    public const CONTINUOS = 1;
+    public const NOMINAL = 2;
+    public const EPSILON = 1e-10;
 
     /**
      * @var array
@@ -57,7 +57,7 @@ class NaiveBayes implements Classifier
      */
     private $labels = [];
 
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         $this->samples = array_merge($this->samples, $samples);
         $this->targets = array_merge($this->targets, $targets);
@@ -77,7 +77,7 @@ class NaiveBayes implements Classifier
      * Calculates vital statistics for each label & feature. Stores these
      * values in private array in order to avoid repeated calculation
      */
-    private function calculateStatistics(string $label, array $samples)
+    private function calculateStatistics(string $label, array $samples): void
     {
         $this->std[$label] = array_fill(0, $this->featureCount, 0);
         $this->mean[$label] = array_fill(0, $this->featureCount, 0);
diff --git a/src/Phpml/Classification/SVC.php b/src/Phpml/Classification/SVC.php
index 8cabcbc..de71bbe 100644
--- a/src/Phpml/Classification/SVC.php
+++ b/src/Phpml/Classification/SVC.php
@@ -14,7 +14,7 @@ class SVC extends SupportVectorMachine implements Classifier
         int $kernel = Kernel::LINEAR,
         float $cost = 1.0,
         int $degree = 3,
-        float $gamma = null,
+        ?float $gamma = null,
         float $coef0 = 0.0,
         float $tolerance = 0.001,
         int $cacheSize = 100,
diff --git a/src/Phpml/Classification/WeightedClassifier.php b/src/Phpml/Classification/WeightedClassifier.php
index 0a5f192..c9b1f97 100644
--- a/src/Phpml/Classification/WeightedClassifier.php
+++ b/src/Phpml/Classification/WeightedClassifier.php
@@ -14,7 +14,7 @@ abstract class WeightedClassifier implements Classifier
     /**
      * Sets the array including a weight for each sample
      */
-    public function setSampleWeights(array $weights)
+    public function setSampleWeights(array $weights): void
     {
         $this->weights = $weights;
     }
diff --git a/src/Phpml/Clustering/DBSCAN.php b/src/Phpml/Clustering/DBSCAN.php
index 70cf302..1968b83 100644
--- a/src/Phpml/Clustering/DBSCAN.php
+++ b/src/Phpml/Clustering/DBSCAN.php
@@ -24,7 +24,7 @@ class DBSCAN implements Clusterer
      */
     private $distanceMetric;
 
-    public function __construct(float $epsilon = 0.5, int $minSamples = 3, Distance $distanceMetric = null)
+    public function __construct(float $epsilon = 0.5, int $minSamples = 3, ?Distance $distanceMetric = null)
     {
         if (null === $distanceMetric) {
             $distanceMetric = new Euclidean();
diff --git a/src/Phpml/Clustering/FuzzyCMeans.php b/src/Phpml/Clustering/FuzzyCMeans.php
index d14c0be..6eccea0 100644
--- a/src/Phpml/Clustering/FuzzyCMeans.php
+++ b/src/Phpml/Clustering/FuzzyCMeans.php
@@ -71,7 +71,7 @@ class FuzzyCMeans implements Clusterer
         $this->maxIterations = $maxIterations;
     }
 
-    protected function initClusters()
+    protected function initClusters(): void
     {
         // Membership array is a matrix of cluster number by sample counts
         // We initilize the membership array with random values
@@ -80,7 +80,7 @@ class FuzzyCMeans implements Clusterer
         $this->updateClusters();
     }
 
-    protected function generateRandomMembership(int $rows, int $cols)
+    protected function generateRandomMembership(int $rows, int $cols): void
     {
         $this->membership = [];
         for ($i = 0; $i < $rows; ++$i) {
@@ -98,7 +98,7 @@ class FuzzyCMeans implements Clusterer
         }
     }
 
-    protected function updateClusters()
+    protected function updateClusters(): void
     {
         $dim = $this->space->getDimension();
         if (!$this->clusters) {
@@ -136,7 +136,7 @@ class FuzzyCMeans implements Clusterer
         return $sum;
     }
 
-    protected function updateMembershipMatrix()
+    protected function updateMembershipMatrix(): void
     {
         for ($i = 0; $i < $this->clustersNumber; ++$i) {
             for ($k = 0; $k < $this->sampleCount; ++$k) {
diff --git a/src/Phpml/Clustering/KMeans.php b/src/Phpml/Clustering/KMeans.php
index 0a776a4..a4e85bc 100644
--- a/src/Phpml/Clustering/KMeans.php
+++ b/src/Phpml/Clustering/KMeans.php
@@ -9,8 +9,8 @@ use Phpml\Exception\InvalidArgumentException;
 
 class KMeans implements Clusterer
 {
-    const INIT_RANDOM = 1;
-    const INIT_KMEANS_PLUS_PLUS = 2;
+    public const INIT_RANDOM = 1;
+    public const INIT_KMEANS_PLUS_PLUS = 2;
 
     /**
      * @var int
diff --git a/src/Phpml/Clustering/KMeans/Cluster.php b/src/Phpml/Clustering/KMeans/Cluster.php
index b7c7ecf..22545b6 100644
--- a/src/Phpml/Clustering/KMeans/Cluster.php
+++ b/src/Phpml/Clustering/KMeans/Cluster.php
@@ -64,17 +64,17 @@ class Cluster extends Point implements IteratorAggregate, Countable
         return $point;
     }
 
-    public function attachAll(SplObjectStorage $points)
+    public function attachAll(SplObjectStorage $points): void
     {
         $this->points->addAll($points);
     }
 
-    public function detachAll(SplObjectStorage $points)
+    public function detachAll(SplObjectStorage $points): void
     {
         $this->points->removeAll($points);
     }
 
-    public function updateCentroid()
+    public function updateCentroid(): void
     {
         if (!$count = count($this->points)) {
             return;
@@ -109,7 +109,7 @@ class Cluster extends Point implements IteratorAggregate, Countable
         return count($this->points);
     }
 
-    public function setCoordinates(array $newCoordinates)
+    public function setCoordinates(array $newCoordinates): void
     {
         $this->coordinates = $newCoordinates;
     }
diff --git a/src/Phpml/Clustering/KMeans/Point.php b/src/Phpml/Clustering/KMeans/Point.php
index 08fa11e..f90de8a 100644
--- a/src/Phpml/Clustering/KMeans/Point.php
+++ b/src/Phpml/Clustering/KMeans/Point.php
@@ -93,7 +93,7 @@ class Point implements ArrayAccess
      * @param mixed $offset
      * @param mixed $value
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         $this->coordinates[$offset] = $value;
     }
@@ -101,7 +101,7 @@ class Point implements ArrayAccess
     /**
      * @param mixed $offset
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         unset($this->coordinates[$offset]);
     }
diff --git a/src/Phpml/Clustering/KMeans/Space.php b/src/Phpml/Clustering/KMeans/Space.php
index bf2a345..4412d53 100644
--- a/src/Phpml/Clustering/KMeans/Space.php
+++ b/src/Phpml/Clustering/KMeans/Space.php
@@ -47,7 +47,7 @@ class Space extends SplObjectStorage
     /**
      * @param null  $data
      */
-    public function addPoint(array $coordinates, $data = null)
+    public function addPoint(array $coordinates, $data = null): void
     {
         $this->attach($this->newPoint($coordinates), $data);
     }
@@ -56,7 +56,7 @@ class Space extends SplObjectStorage
      * @param Point $point
      * @param null  $data
      */
-    public function attach($point, $data = null)
+    public function attach($point, $data = null): void
     {
         if (!$point instanceof Point) {
             throw new InvalidArgumentException('can only attach points to spaces');
@@ -180,7 +180,7 @@ class Space extends SplObjectStorage
     private function initializeRandomClusters(int $clustersNumber) : array
     {
         $clusters = [];
-        list($min, $max) = $this->getBoundaries();
+        [$min, $max] = $this->getBoundaries();
 
         for ($n = 0; $n < $clustersNumber; ++$n) {
             $clusters[] = new Cluster($this, $this->getRandomPoint($min, $max)->getCoordinates());
diff --git a/src/Phpml/CrossValidation/RandomSplit.php b/src/Phpml/CrossValidation/RandomSplit.php
index 7ec1875..8507ee5 100644
--- a/src/Phpml/CrossValidation/RandomSplit.php
+++ b/src/Phpml/CrossValidation/RandomSplit.php
@@ -8,7 +8,7 @@ use Phpml\Dataset\Dataset;
 
 class RandomSplit extends Split
 {
-    protected function splitDataset(Dataset $dataset, float $testSize)
+    protected function splitDataset(Dataset $dataset, float $testSize): void
     {
         $samples = $dataset->getSamples();
         $labels = $dataset->getTargets();
diff --git a/src/Phpml/CrossValidation/Split.php b/src/Phpml/CrossValidation/Split.php
index 77eab3c..e485ffb 100644
--- a/src/Phpml/CrossValidation/Split.php
+++ b/src/Phpml/CrossValidation/Split.php
@@ -29,7 +29,7 @@ abstract class Split
      */
     protected $testLabels = [];
 
-    public function __construct(Dataset $dataset, float $testSize = 0.3, int $seed = null)
+    public function __construct(Dataset $dataset, float $testSize = 0.3, ?int $seed = null)
     {
         if (0 >= $testSize || 1 <= $testSize) {
             throw InvalidArgumentException::percentNotInRange('testSize');
@@ -61,7 +61,7 @@ abstract class Split
         return $this->testLabels;
     }
 
-    protected function seedGenerator(int $seed = null)
+    protected function seedGenerator(?int $seed = null): void
     {
         if (null === $seed) {
             mt_srand();
diff --git a/src/Phpml/CrossValidation/StratifiedRandomSplit.php b/src/Phpml/CrossValidation/StratifiedRandomSplit.php
index 41f5d34..153cb8f 100644
--- a/src/Phpml/CrossValidation/StratifiedRandomSplit.php
+++ b/src/Phpml/CrossValidation/StratifiedRandomSplit.php
@@ -9,7 +9,7 @@ use Phpml\Dataset\Dataset;
 
 class StratifiedRandomSplit extends RandomSplit
 {
-    protected function splitDataset(Dataset $dataset, float $testSize)
+    protected function splitDataset(Dataset $dataset, float $testSize): void
     {
         $datasets = $this->splitByTarget($dataset);
 
diff --git a/src/Phpml/Dataset/FilesDataset.php b/src/Phpml/Dataset/FilesDataset.php
index 73bc4b0..ca04a3e 100644
--- a/src/Phpml/Dataset/FilesDataset.php
+++ b/src/Phpml/Dataset/FilesDataset.php
@@ -17,14 +17,14 @@ class FilesDataset extends ArrayDataset
         $this->scanRootPath($rootPath);
     }
 
-    private function scanRootPath(string $rootPath)
+    private function scanRootPath(string $rootPath): void
     {
         foreach (glob($rootPath.DIRECTORY_SEPARATOR.'*', GLOB_ONLYDIR) as $dir) {
             $this->scanDir($dir);
         }
     }
 
-    private function scanDir(string $dir)
+    private function scanDir(string $dir): void
     {
         $target = basename($dir);
 
diff --git a/src/Phpml/DimensionReduction/EigenTransformerBase.php b/src/Phpml/DimensionReduction/EigenTransformerBase.php
index df18d11..a6352ba 100644
--- a/src/Phpml/DimensionReduction/EigenTransformerBase.php
+++ b/src/Phpml/DimensionReduction/EigenTransformerBase.php
@@ -48,7 +48,7 @@ abstract class EigenTransformerBase
      * top eigenVectors along with the largest eigenValues. The total explained variance
      * of these eigenVectors will be no less than desired $totalVariance value
      */
-    protected function eigenDecomposition(array $matrix)
+    protected function eigenDecomposition(array $matrix): void
     {
         $eig = new EigenvalueDecomposition($matrix);
         $eigVals = $eig->getRealEigenvalues();
diff --git a/src/Phpml/DimensionReduction/KernelPCA.php b/src/Phpml/DimensionReduction/KernelPCA.php
index 1c7cecb..d11e1a6 100644
--- a/src/Phpml/DimensionReduction/KernelPCA.php
+++ b/src/Phpml/DimensionReduction/KernelPCA.php
@@ -10,10 +10,10 @@ use Phpml\Math\Matrix;
 
 class KernelPCA extends PCA
 {
-    const KERNEL_RBF = 1;
-    const KERNEL_SIGMOID = 2;
-    const KERNEL_LAPLACIAN = 3;
-    const KERNEL_LINEAR = 4;
+    public const KERNEL_RBF = 1;
+    public const KERNEL_SIGMOID = 2;
+    public const KERNEL_LAPLACIAN = 3;
+    public const KERNEL_LINEAR = 4;
 
     /**
      * Selected kernel function
@@ -50,7 +50,7 @@ class KernelPCA extends PCA
      *
      * @throws \Exception
      */
-    public function __construct(int $kernel = self::KERNEL_RBF, float $totalVariance = null, int $numFeatures = null, float $gamma = null)
+    public function __construct(int $kernel = self::KERNEL_RBF, ?float $totalVariance = null, ?int $numFeatures = null, ?float $gamma = null)
     {
         $availableKernels = [self::KERNEL_RBF, self::KERNEL_SIGMOID, self::KERNEL_LAPLACIAN, self::KERNEL_LINEAR];
         if (!in_array($kernel, $availableKernels)) {
diff --git a/src/Phpml/DimensionReduction/LDA.php b/src/Phpml/DimensionReduction/LDA.php
index 8a94f46..26b2324 100644
--- a/src/Phpml/DimensionReduction/LDA.php
+++ b/src/Phpml/DimensionReduction/LDA.php
@@ -47,7 +47,7 @@ class LDA extends EigenTransformerBase
      *
      * @throws \Exception
      */
-    public function __construct(float $totalVariance = null, int $numFeatures = null)
+    public function __construct(?float $totalVariance = null, ?int $numFeatures = null)
     {
         if ($totalVariance !== null && ($totalVariance < 0.1 || $totalVariance > 0.99)) {
             throw new \Exception('Total variance can be a value between 0.1 and 0.99');
diff --git a/src/Phpml/DimensionReduction/PCA.php b/src/Phpml/DimensionReduction/PCA.php
index f5cb219..25b7186 100644
--- a/src/Phpml/DimensionReduction/PCA.php
+++ b/src/Phpml/DimensionReduction/PCA.php
@@ -32,7 +32,7 @@ class PCA extends EigenTransformerBase
      *
      * @throws \Exception
      */
-    public function __construct(float $totalVariance = null, int $numFeatures = null)
+    public function __construct(?float $totalVariance = null, ?int $numFeatures = null)
     {
         if ($totalVariance !== null && ($totalVariance < 0.1 || $totalVariance > 0.99)) {
             throw new \Exception('Total variance can be a value between 0.1 and 0.99');
@@ -73,7 +73,7 @@ class PCA extends EigenTransformerBase
         return $this->reduce($data);
     }
 
-    protected function calculateMeans(array $data, int $n)
+    protected function calculateMeans(array $data, int $n): void
     {
         // Calculate means for each dimension
         $this->means = [];
diff --git a/src/Phpml/FeatureExtraction/TfIdfTransformer.php b/src/Phpml/FeatureExtraction/TfIdfTransformer.php
index c5e1e59..6efd90f 100644
--- a/src/Phpml/FeatureExtraction/TfIdfTransformer.php
+++ b/src/Phpml/FeatureExtraction/TfIdfTransformer.php
@@ -13,14 +13,14 @@ class TfIdfTransformer implements Transformer
      */
     private $idf;
 
-    public function __construct(array $samples = null)
+    public function __construct(?array $samples = null)
     {
         if ($samples) {
             $this->fit($samples);
         }
     }
 
-    public function fit(array $samples)
+    public function fit(array $samples): void
     {
         $this->countTokensFrequency($samples);
 
@@ -30,7 +30,7 @@ class TfIdfTransformer implements Transformer
         }
     }
 
-    public function transform(array &$samples)
+    public function transform(array &$samples): void
     {
         foreach ($samples as &$sample) {
             foreach ($sample as $index => &$feature) {
@@ -39,7 +39,7 @@ class TfIdfTransformer implements Transformer
         }
     }
 
-    private function countTokensFrequency(array $samples)
+    private function countTokensFrequency(array $samples): void
     {
         $this->idf = array_fill_keys(array_keys($samples[0]), 0);
 
diff --git a/src/Phpml/FeatureExtraction/TokenCountVectorizer.php b/src/Phpml/FeatureExtraction/TokenCountVectorizer.php
index eb7a4ac..e00fc69 100644
--- a/src/Phpml/FeatureExtraction/TokenCountVectorizer.php
+++ b/src/Phpml/FeatureExtraction/TokenCountVectorizer.php
@@ -34,7 +34,7 @@ class TokenCountVectorizer implements Transformer
      */
     private $frequencies;
 
-    public function __construct(Tokenizer $tokenizer, StopWords $stopWords = null, float $minDF = 0.0)
+    public function __construct(Tokenizer $tokenizer, ?StopWords $stopWords = null, float $minDF = 0.0)
     {
         $this->tokenizer = $tokenizer;
         $this->stopWords = $stopWords;
@@ -44,12 +44,12 @@ class TokenCountVectorizer implements Transformer
         $this->frequencies = [];
     }
 
-    public function fit(array $samples)
+    public function fit(array $samples): void
     {
         $this->buildVocabulary($samples);
     }
 
-    public function transform(array &$samples)
+    public function transform(array &$samples): void
     {
         foreach ($samples as &$sample) {
             $this->transformSample($sample);
@@ -63,7 +63,7 @@ class TokenCountVectorizer implements Transformer
         return array_flip($this->vocabulary);
     }
 
-    private function buildVocabulary(array &$samples)
+    private function buildVocabulary(array &$samples): void
     {
         foreach ($samples as $index => $sample) {
             $tokens = $this->tokenizer->tokenize($sample);
@@ -73,7 +73,7 @@ class TokenCountVectorizer implements Transformer
         }
     }
 
-    private function transformSample(string &$sample)
+    private function transformSample(string &$sample): void
     {
         $counts = [];
         $tokens = $this->tokenizer->tokenize($sample);
@@ -113,7 +113,7 @@ class TokenCountVectorizer implements Transformer
         return $this->vocabulary[$token] ?? false;
     }
 
-    private function addTokenToVocabulary(string $token)
+    private function addTokenToVocabulary(string $token): void
     {
         if ($this->isStopWord($token)) {
             return;
@@ -129,7 +129,7 @@ class TokenCountVectorizer implements Transformer
         return $this->stopWords && $this->stopWords->isStopWord($token);
     }
 
-    private function updateFrequency(string $token)
+    private function updateFrequency(string $token): void
     {
         if (!isset($this->frequencies[$token])) {
             $this->frequencies[$token] = 0;
@@ -138,7 +138,7 @@ class TokenCountVectorizer implements Transformer
         ++$this->frequencies[$token];
     }
 
-    private function checkDocumentFrequency(array &$samples)
+    private function checkDocumentFrequency(array &$samples): void
     {
         if ($this->minDF > 0) {
             $beyondMinimum = $this->getBeyondMinimumIndexes(count($samples));
@@ -148,7 +148,7 @@ class TokenCountVectorizer implements Transformer
         }
     }
 
-    private function resetBeyondMinimum(array &$sample, array $beyondMinimum)
+    private function resetBeyondMinimum(array &$sample, array $beyondMinimum): void
     {
         foreach ($beyondMinimum as $index) {
             $sample[$index] = 0;
diff --git a/src/Phpml/Helper/OneVsRest.php b/src/Phpml/Helper/OneVsRest.php
index 5ae126c..15d62d8 100644
--- a/src/Phpml/Helper/OneVsRest.php
+++ b/src/Phpml/Helper/OneVsRest.php
@@ -28,7 +28,7 @@ trait OneVsRest
     /**
      * Train a binary classifier in the OvR style
      */
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         // Clears previous stuff.
         $this->reset();
@@ -36,7 +36,7 @@ trait OneVsRest
         $this->trainBylabel($samples, $targets);
     }
 
-    protected function trainByLabel(array $samples, array $targets, array $allLabels = [])
+    protected function trainByLabel(array $samples, array $targets, array $allLabels = []): void
     {
         // Overwrites the current value if it exist. $allLabels must be provided for each partialTrain run.
         if (!empty($allLabels)) {
@@ -63,7 +63,7 @@ trait OneVsRest
                     $this->classifiers[$label] = $this->getClassifierCopy();
                 }
 
-                list($binarizedTargets, $classifierLabels) = $this->binarizeTargets($targets, $label);
+                [$binarizedTargets, $classifierLabels] = $this->binarizeTargets($targets, $label);
                 $this->classifiers[$label]->trainBinary($samples, $binarizedTargets, $classifierLabels);
             }
         }
@@ -80,7 +80,7 @@ trait OneVsRest
     /**
      * Resets the classifier and the vars internally used by OneVsRest to create multiple classifiers.
      */
-    public function reset()
+    public function reset(): void
     {
         $this->classifiers = [];
         $this->allLabels = [];
@@ -158,7 +158,7 @@ trait OneVsRest
      *
      * @return void
      */
-    abstract protected function resetBinary();
+    abstract protected function resetBinary(): void;
 
     /**
      * Each classifier that make use of OvR approach should be able to
diff --git a/src/Phpml/Helper/Optimizer/ConjugateGradient.php b/src/Phpml/Helper/Optimizer/ConjugateGradient.php
index 7333b9a..c119eae 100644
--- a/src/Phpml/Helper/Optimizer/ConjugateGradient.php
+++ b/src/Phpml/Helper/Optimizer/ConjugateGradient.php
@@ -61,7 +61,7 @@ class ConjugateGradient extends GD
      */
     protected function gradient(array $theta) : array
     {
-        list(, $gradient) = parent::gradient($theta);
+        [, $gradient] = parent::gradient($theta);
 
         return $gradient;
     }
@@ -71,7 +71,7 @@ class ConjugateGradient extends GD
      */
     protected function cost(array $theta) : float
     {
-        list($cost) = parent::gradient($theta);
+        [$cost] = parent::gradient($theta);
 
         return array_sum($cost) / $this->sampleCount;
     }
diff --git a/src/Phpml/Helper/Optimizer/GD.php b/src/Phpml/Helper/Optimizer/GD.php
index ae3c6e2..38b4253 100644
--- a/src/Phpml/Helper/Optimizer/GD.php
+++ b/src/Phpml/Helper/Optimizer/GD.php
@@ -31,7 +31,7 @@ class GD extends StochasticGD
             $theta = $this->theta;
 
             // Calculate update terms for each sample
-            list($errors, $updates, $totalPenalty) = $this->gradient($theta);
+            [$errors, $updates, $totalPenalty] = $this->gradient($theta);
 
             $this->updateWeightsWithUpdates($updates, $totalPenalty);
 
@@ -61,7 +61,7 @@ class GD extends StochasticGD
             $target = $this->targets[$index];
 
             $result = ($this->gradientCb)($theta, $sample, $target);
-            list($cost, $grad, $penalty) = array_pad($result, 3, 0);
+            [$cost, $grad, $penalty] = array_pad($result, 3, 0);
 
             $costs[] = $cost;
             $gradient[] = $grad;
@@ -73,7 +73,7 @@ class GD extends StochasticGD
         return [$costs, $gradient, $totalPenalty];
     }
 
-    protected function updateWeightsWithUpdates(array $updates, float $penalty)
+    protected function updateWeightsWithUpdates(array $updates, float $penalty): void
     {
         // Updates all weights at once
         for ($i = 0; $i <= $this->dimensions; ++$i) {
@@ -96,7 +96,7 @@ class GD extends StochasticGD
     /**
      * Clears the optimizer internal vars after the optimization process.
      */
-    protected function clear()
+    protected function clear(): void
     {
         $this->sampleCount = null;
         parent::clear();
diff --git a/src/Phpml/Helper/Optimizer/StochasticGD.php b/src/Phpml/Helper/Optimizer/StochasticGD.php
index 0a622d4..f1b0979 100644
--- a/src/Phpml/Helper/Optimizer/StochasticGD.php
+++ b/src/Phpml/Helper/Optimizer/StochasticGD.php
@@ -191,7 +191,7 @@ class StochasticGD extends Optimizer
 
             $result = ($this->gradientCb)($theta, $sample, $target);
 
-            list($error, $gradient, $penalty) = array_pad($result, 3, 0);
+            [$error, $gradient, $penalty] = array_pad($result, 3, 0);
 
             // Update bias
             $this->theta[0] -= $this->learningRate * $gradient;
@@ -249,7 +249,7 @@ class StochasticGD extends Optimizer
     /**
      * Clears the optimizer internal vars after the optimization process.
      */
-    protected function clear()
+    protected function clear(): void
     {
         $this->samples = [];
         $this->targets = [];
diff --git a/src/Phpml/Helper/Trainable.php b/src/Phpml/Helper/Trainable.php
index 3d011ac..86ffaf1 100644
--- a/src/Phpml/Helper/Trainable.php
+++ b/src/Phpml/Helper/Trainable.php
@@ -20,7 +20,7 @@ trait Trainable
      * @param array $samples
      * @param array $targets
      */
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         $this->samples = array_merge($this->samples, $samples);
         $this->targets = array_merge($this->targets, $targets);
diff --git a/src/Phpml/Math/LinearAlgebra/EigenvalueDecomposition.php b/src/Phpml/Math/LinearAlgebra/EigenvalueDecomposition.php
index b76ddb9..6261d26 100644
--- a/src/Phpml/Math/LinearAlgebra/EigenvalueDecomposition.php
+++ b/src/Phpml/Math/LinearAlgebra/EigenvalueDecomposition.php
@@ -119,7 +119,7 @@ class EigenvalueDecomposition
     /**
      *	Symmetric Householder reduction to tridiagonal form.
      */
-    private function tred2()
+    private function tred2(): void
     {
         //  This is derived from the Algol procedures tred2 by
         //  Bowdler, Martin, Reinsch, and Wilkinson, Handbook for
@@ -236,7 +236,7 @@ class EigenvalueDecomposition
      *	Auto. Comp., Vol.ii-Linear Algebra, and the corresponding
      *	Fortran subroutine in EISPACK.
      */
-    private function tql2()
+    private function tql2(): void
     {
         for ($i = 1; $i < $this->n; ++$i) {
             $this->e[$i - 1] = $this->e[$i];
@@ -343,7 +343,7 @@ class EigenvalueDecomposition
      *	Vol.ii-Linear Algebra, and the corresponding
      *	Fortran subroutines in EISPACK.
      */
-    private function orthes()
+    private function orthes(): void
     {
         $low = 0;
         $high = $this->n - 1;
@@ -428,7 +428,7 @@ class EigenvalueDecomposition
      * @param int|float $yr
      * @param int|float $yi
      */
-    private function cdiv($xr, $xi, $yr, $yi)
+    private function cdiv($xr, $xi, $yr, $yi): void
     {
         if (abs($yr) > abs($yi)) {
             $r = $yi / $yr;
@@ -451,7 +451,7 @@ class EigenvalueDecomposition
      *	Vol.ii-Linear Algebra, and the corresponding
      *	Fortran subroutine in EISPACK.
      */
-    private function hqr2()
+    private function hqr2(): void
     {
         //  Initialize
         $nn = $this->n;
diff --git a/src/Phpml/Math/Statistic/Covariance.php b/src/Phpml/Math/Statistic/Covariance.php
index 2c72e6c..627a8a6 100644
--- a/src/Phpml/Math/Statistic/Covariance.php
+++ b/src/Phpml/Math/Statistic/Covariance.php
@@ -13,7 +13,7 @@ class Covariance
      *
      * @throws InvalidArgumentException
      */
-    public static function fromXYArrays(array $x, array $y, bool $sample = true, float $meanX = null, float $meanY = null) : float
+    public static function fromXYArrays(array $x, array $y, bool $sample = true, ?float $meanX = null, ?float $meanY = null) : float
     {
         if (empty($x) || empty($y)) {
             throw InvalidArgumentException::arrayCantBeEmpty();
@@ -51,7 +51,7 @@ class Covariance
      * @throws InvalidArgumentException
      * @throws \Exception
      */
-    public static function fromDataset(array $data, int $i, int $k, bool $sample = true, float $meanX = null, float $meanY = null) : float
+    public static function fromDataset(array $data, int $i, int $k, bool $sample = true, ?float $meanX = null, ?float $meanY = null) : float
     {
         if (empty($data)) {
             throw InvalidArgumentException::arrayCantBeEmpty();
@@ -112,7 +112,7 @@ class Covariance
      *
      * @param array|null $means
      */
-    public static function covarianceMatrix(array $data, array $means = null) : array
+    public static function covarianceMatrix(array $data, ?array $means = null) : array
     {
         $n = count($data[0]);
 
diff --git a/src/Phpml/Math/Statistic/Mean.php b/src/Phpml/Math/Statistic/Mean.php
index 50b3788..eb1baef 100644
--- a/src/Phpml/Math/Statistic/Mean.php
+++ b/src/Phpml/Math/Statistic/Mean.php
@@ -56,7 +56,7 @@ class Mean
     /**
      * @throws InvalidArgumentException
      */
-    private static function checkArrayLength(array $array)
+    private static function checkArrayLength(array $array): void
     {
         if (empty($array)) {
             throw InvalidArgumentException::arrayCantBeEmpty();
diff --git a/src/Phpml/Metric/ClassificationReport.php b/src/Phpml/Metric/ClassificationReport.php
index f4fdaee..ae4c11a 100644
--- a/src/Phpml/Metric/ClassificationReport.php
+++ b/src/Phpml/Metric/ClassificationReport.php
@@ -76,7 +76,7 @@ class ClassificationReport
         return $this->average;
     }
 
-    private function computeMetrics(array $truePositive, array $falsePositive, array $falseNegative)
+    private function computeMetrics(array $truePositive, array $falsePositive, array $falseNegative): void
     {
         foreach ($truePositive as $label => $tp) {
             $this->precision[$label] = $this->computePrecision($tp, $falsePositive[$label]);
@@ -85,7 +85,7 @@ class ClassificationReport
         }
     }
 
-    private function computeAverage()
+    private function computeAverage(): void
     {
         foreach (['precision', 'recall', 'f1score'] as $metric) {
             $values = array_filter($this->{$metric});
diff --git a/src/Phpml/Metric/ConfusionMatrix.php b/src/Phpml/Metric/ConfusionMatrix.php
index 5c7bc0b..0f0b738 100644
--- a/src/Phpml/Metric/ConfusionMatrix.php
+++ b/src/Phpml/Metric/ConfusionMatrix.php
@@ -6,7 +6,7 @@ namespace Phpml\Metric;
 
 class ConfusionMatrix
 {
-    public static function compute(array $actualLabels, array $predictedLabels, array $labels = null) : array
+    public static function compute(array $actualLabels, array $predictedLabels, ?array $labels = null) : array
     {
         $labels = $labels ? array_flip($labels) : self::getUniqueLabels($actualLabels);
         $matrix = self::generateMatrixWithZeros($labels);
diff --git a/src/Phpml/ModelManager.php b/src/Phpml/ModelManager.php
index 28594be..2fa14f5 100644
--- a/src/Phpml/ModelManager.php
+++ b/src/Phpml/ModelManager.php
@@ -9,7 +9,7 @@ use Phpml\Exception\SerializeException;
 
 class ModelManager
 {
-    public function saveToFile(Estimator $estimator, string $filepath)
+    public function saveToFile(Estimator $estimator, string $filepath): void
     {
         if (!is_writable(dirname($filepath))) {
             throw FileException::cantSaveFile(basename($filepath));
diff --git a/src/Phpml/NeuralNetwork/Layer.php b/src/Phpml/NeuralNetwork/Layer.php
index 524bf35..c70bdb3 100644
--- a/src/Phpml/NeuralNetwork/Layer.php
+++ b/src/Phpml/NeuralNetwork/Layer.php
@@ -17,7 +17,7 @@ class Layer
     /**
      * @throws InvalidArgumentException
      */
-    public function __construct(int $nodesNumber = 0, string $nodeClass = Neuron::class, ActivationFunction $activationFunction = null)
+    public function __construct(int $nodesNumber = 0, string $nodeClass = Neuron::class, ?ActivationFunction $activationFunction = null)
     {
         if (!in_array(Node::class, class_implements($nodeClass))) {
             throw InvalidArgumentException::invalidLayerNodeClass();
@@ -33,7 +33,7 @@ class Layer
      *
      * @return Neuron
      */
-    private function createNode(string $nodeClass, ActivationFunction $activationFunction = null)
+    private function createNode(string $nodeClass, ?ActivationFunction $activationFunction = null)
     {
         if (Neuron::class == $nodeClass) {
             return new Neuron($activationFunction);
@@ -42,7 +42,7 @@ class Layer
         return new $nodeClass();
     }
 
-    public function addNode(Node $node)
+    public function addNode(Node $node): void
     {
         $this->nodes[] = $node;
     }
diff --git a/src/Phpml/NeuralNetwork/Network/LayeredNetwork.php b/src/Phpml/NeuralNetwork/Network/LayeredNetwork.php
index 1234c41..3baa5ac 100644
--- a/src/Phpml/NeuralNetwork/Network/LayeredNetwork.php
+++ b/src/Phpml/NeuralNetwork/Network/LayeredNetwork.php
@@ -16,7 +16,7 @@ abstract class LayeredNetwork implements Network
      */
     protected $layers;
 
-    public function addLayer(Layer $layer)
+    public function addLayer(Layer $layer): void
     {
         $this->layers[] = $layer;
     }
@@ -29,7 +29,7 @@ abstract class LayeredNetwork implements Network
         return $this->layers;
     }
 
-    public function removeLayers()
+    public function removeLayers(): void
     {
         unset($this->layers);
     }
diff --git a/src/Phpml/NeuralNetwork/Network/MultilayerPerceptron.php b/src/Phpml/NeuralNetwork/Network/MultilayerPerceptron.php
index 530fbef..9ef3f73 100644
--- a/src/Phpml/NeuralNetwork/Network/MultilayerPerceptron.php
+++ b/src/Phpml/NeuralNetwork/Network/MultilayerPerceptron.php
@@ -58,7 +58,7 @@ abstract class MultilayerPerceptron extends LayeredNetwork implements Estimator,
     /**
      * @throws InvalidArgumentException
      */
-    public function __construct(int $inputLayerFeatures, array $hiddenLayers, array $classes, int $iterations = 10000, ActivationFunction $activationFunction = null, int $theta = 1)
+    public function __construct(int $inputLayerFeatures, array $hiddenLayers, array $classes, int $iterations = 10000, ?ActivationFunction $activationFunction = null, int $theta = 1)
     {
         if (empty($hiddenLayers)) {
             throw InvalidArgumentException::invalidLayersNumber();
@@ -78,7 +78,7 @@ abstract class MultilayerPerceptron extends LayeredNetwork implements Estimator,
         $this->initNetwork();
     }
 
-    private function initNetwork()
+    private function initNetwork(): void
     {
         $this->addInputLayer($this->inputLayerFeatures);
         $this->addNeuronLayers($this->hiddenLayers, $this->activationFunction);
@@ -90,7 +90,7 @@ abstract class MultilayerPerceptron extends LayeredNetwork implements Estimator,
         $this->backpropagation = new Backpropagation($this->theta);
     }
 
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         $this->reset();
         $this->initNetwork();
@@ -100,7 +100,7 @@ abstract class MultilayerPerceptron extends LayeredNetwork implements Estimator,
     /**
      * @throws InvalidArgumentException
      */
-    public function partialTrain(array $samples, array $targets, array $classes = [])
+    public function partialTrain(array $samples, array $targets, array $classes = []): void
     {
         if (!empty($classes) && array_values($classes) !== $this->classes) {
             // We require the list of classes in the constructor.
@@ -122,24 +122,24 @@ abstract class MultilayerPerceptron extends LayeredNetwork implements Estimator,
      */
     abstract protected function predictSample(array $sample);
 
-    protected function reset()
+    protected function reset(): void
     {
         $this->removeLayers();
     }
 
-    private function addInputLayer(int $nodes)
+    private function addInputLayer(int $nodes): void
     {
         $this->addLayer(new Layer($nodes, Input::class));
     }
 
-    private function addNeuronLayers(array $layers, ActivationFunction $activationFunction = null)
+    private function addNeuronLayers(array $layers, ?ActivationFunction $activationFunction = null): void
     {
         foreach ($layers as $neurons) {
             $this->addLayer(new Layer($neurons, Neuron::class, $activationFunction));
         }
     }
 
-    private function generateSynapses()
+    private function generateSynapses(): void
     {
         $layersNumber = count($this->layers) - 1;
         for ($i = 0; $i < $layersNumber; ++$i) {
@@ -149,7 +149,7 @@ abstract class MultilayerPerceptron extends LayeredNetwork implements Estimator,
         }
     }
 
-    private function addBiasNodes()
+    private function addBiasNodes(): void
     {
         $biasLayers = count($this->layers) - 1;
         for ($i = 0; $i < $biasLayers; ++$i) {
@@ -157,7 +157,7 @@ abstract class MultilayerPerceptron extends LayeredNetwork implements Estimator,
         }
     }
 
-    private function generateLayerSynapses(Layer $nextLayer, Layer $currentLayer)
+    private function generateLayerSynapses(Layer $nextLayer, Layer $currentLayer): void
     {
         foreach ($nextLayer->getNodes() as $nextNeuron) {
             if ($nextNeuron instanceof Neuron) {
@@ -166,14 +166,14 @@ abstract class MultilayerPerceptron extends LayeredNetwork implements Estimator,
         }
     }
 
-    private function generateNeuronSynapses(Layer $currentLayer, Neuron $nextNeuron)
+    private function generateNeuronSynapses(Layer $currentLayer, Neuron $nextNeuron): void
     {
         foreach ($currentLayer->getNodes() as $currentNeuron) {
             $nextNeuron->addSynapse(new Synapse($currentNeuron));
         }
     }
 
-    private function trainSamples(array $samples, array $targets)
+    private function trainSamples(array $samples, array $targets): void
     {
         foreach ($targets as $key => $target) {
             $this->trainSample($samples[$key], $target);
diff --git a/src/Phpml/NeuralNetwork/Node/Input.php b/src/Phpml/NeuralNetwork/Node/Input.php
index 569b303..8ff78ea 100644
--- a/src/Phpml/NeuralNetwork/Node/Input.php
+++ b/src/Phpml/NeuralNetwork/Node/Input.php
@@ -23,7 +23,7 @@ class Input implements Node
         return $this->input;
     }
 
-    public function setInput(float $input)
+    public function setInput(float $input): void
     {
         $this->input = $input;
     }
diff --git a/src/Phpml/NeuralNetwork/Node/Neuron.php b/src/Phpml/NeuralNetwork/Node/Neuron.php
index 520c3b2..096d54f 100644
--- a/src/Phpml/NeuralNetwork/Node/Neuron.php
+++ b/src/Phpml/NeuralNetwork/Node/Neuron.php
@@ -25,14 +25,14 @@ class Neuron implements Node
      */
     protected $output;
 
-    public function __construct(ActivationFunction $activationFunction = null)
+    public function __construct(?ActivationFunction $activationFunction = null)
     {
         $this->activationFunction = $activationFunction ?: new ActivationFunction\Sigmoid();
         $this->synapses = [];
         $this->output = 0;
     }
 
-    public function addSynapse(Synapse $synapse)
+    public function addSynapse(Synapse $synapse): void
     {
         $this->synapses[] = $synapse;
     }
@@ -59,7 +59,7 @@ class Neuron implements Node
         return $this->output;
     }
 
-    public function reset()
+    public function reset(): void
     {
         $this->output = 0;
     }
diff --git a/src/Phpml/NeuralNetwork/Node/Neuron/Synapse.php b/src/Phpml/NeuralNetwork/Node/Neuron/Synapse.php
index adb4b02..0883f4e 100644
--- a/src/Phpml/NeuralNetwork/Node/Neuron/Synapse.php
+++ b/src/Phpml/NeuralNetwork/Node/Neuron/Synapse.php
@@ -21,7 +21,7 @@ class Synapse
     /**
      * @param float|null $weight
      */
-    public function __construct(Node $node, float $weight = null)
+    public function __construct(Node $node, ?float $weight = null)
     {
         $this->node = $node;
         $this->weight = $weight ?: $this->generateRandomWeight();
@@ -37,7 +37,7 @@ class Synapse
         return $this->weight * $this->node->getOutput();
     }
 
-    public function changeWeight(float $delta)
+    public function changeWeight(float $delta): void
     {
         $this->weight += $delta;
     }
diff --git a/src/Phpml/NeuralNetwork/Training/Backpropagation.php b/src/Phpml/NeuralNetwork/Training/Backpropagation.php
index b113454..98683ab 100644
--- a/src/Phpml/NeuralNetwork/Training/Backpropagation.php
+++ b/src/Phpml/NeuralNetwork/Training/Backpropagation.php
@@ -32,7 +32,7 @@ class Backpropagation
     /**
      * @param mixed $targetClass
      */
-    public function backpropagate(array $layers, $targetClass)
+    public function backpropagate(array $layers, $targetClass): void
     {
         $layersNumber = count($layers);
 
diff --git a/src/Phpml/Pipeline.php b/src/Phpml/Pipeline.php
index a49fb27..a72e634 100644
--- a/src/Phpml/Pipeline.php
+++ b/src/Phpml/Pipeline.php
@@ -28,12 +28,12 @@ class Pipeline implements Estimator
         $this->estimator = $estimator;
     }
 
-    public function addTransformer(Transformer $transformer)
+    public function addTransformer(Transformer $transformer): void
     {
         $this->transformers[] = $transformer;
     }
 
-    public function setEstimator(Estimator $estimator)
+    public function setEstimator(Estimator $estimator): void
     {
         $this->estimator = $estimator;
     }
@@ -51,7 +51,7 @@ class Pipeline implements Estimator
         return $this->estimator;
     }
 
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         foreach ($this->transformers as $transformer) {
             $transformer->fit($samples);
@@ -71,7 +71,7 @@ class Pipeline implements Estimator
         return $this->estimator->predict($samples);
     }
 
-    private function transformSamples(array &$samples)
+    private function transformSamples(array &$samples): void
     {
         foreach ($this->transformers as $transformer) {
             $transformer->transform($samples);
diff --git a/src/Phpml/Preprocessing/Imputer.php b/src/Phpml/Preprocessing/Imputer.php
index cc90c46..d2dbfcd 100644
--- a/src/Phpml/Preprocessing/Imputer.php
+++ b/src/Phpml/Preprocessing/Imputer.php
@@ -8,8 +8,8 @@ use Phpml\Preprocessing\Imputer\Strategy;
 
 class Imputer implements Preprocessor
 {
-    const AXIS_COLUMN = 0;
-    const AXIS_ROW = 1;
+    public const AXIS_COLUMN = 0;
+    public const AXIS_ROW = 1;
 
     /**
      * @var mixed
@@ -43,19 +43,19 @@ class Imputer implements Preprocessor
         $this->samples = $samples;
     }
 
-    public function fit(array $samples)
+    public function fit(array $samples): void
     {
         $this->samples = $samples;
     }
 
-    public function transform(array &$samples)
+    public function transform(array &$samples): void
     {
         foreach ($samples as &$sample) {
             $this->preprocessSample($sample);
         }
     }
 
-    private function preprocessSample(array &$sample)
+    private function preprocessSample(array &$sample): void
     {
         foreach ($sample as $column => &$value) {
             if ($value === $this->missingValue) {
diff --git a/src/Phpml/Preprocessing/Normalizer.php b/src/Phpml/Preprocessing/Normalizer.php
index 07777f9..f038345 100644
--- a/src/Phpml/Preprocessing/Normalizer.php
+++ b/src/Phpml/Preprocessing/Normalizer.php
@@ -10,9 +10,9 @@ use Phpml\Math\Statistic\StandardDeviation;
 
 class Normalizer implements Preprocessor
 {
-    const NORM_L1 = 1;
-    const NORM_L2 = 2;
-    const NORM_STD = 3;
+    public const NORM_L1 = 1;
+    public const NORM_L2 = 2;
+    public const NORM_STD = 3;
 
     /**
      * @var int
@@ -46,7 +46,7 @@ class Normalizer implements Preprocessor
         $this->norm = $norm;
     }
 
-    public function fit(array $samples)
+    public function fit(array $samples): void
     {
         if ($this->fitted) {
             return;
@@ -64,7 +64,7 @@ class Normalizer implements Preprocessor
         $this->fitted = true;
     }
 
-    public function transform(array &$samples)
+    public function transform(array &$samples): void
     {
         $methods = [
             self::NORM_L1 => 'normalizeL1',
@@ -80,7 +80,7 @@ class Normalizer implements Preprocessor
         }
     }
 
-    private function normalizeL1(array &$sample)
+    private function normalizeL1(array &$sample): void
     {
         $norm1 = 0;
         foreach ($sample as $feature) {
@@ -97,7 +97,7 @@ class Normalizer implements Preprocessor
         }
     }
 
-    private function normalizeL2(array &$sample)
+    private function normalizeL2(array &$sample): void
     {
         $norm2 = 0;
         foreach ($sample as $feature) {
@@ -114,7 +114,7 @@ class Normalizer implements Preprocessor
         }
     }
 
-    private function normalizeSTD(array &$sample)
+    private function normalizeSTD(array &$sample): void
     {
         foreach ($sample as $i => $val) {
             if ($this->std[$i] != 0) {
diff --git a/src/Phpml/Regression/LeastSquares.php b/src/Phpml/Regression/LeastSquares.php
index 28602cd..f8adcb2 100644
--- a/src/Phpml/Regression/LeastSquares.php
+++ b/src/Phpml/Regression/LeastSquares.php
@@ -30,7 +30,7 @@ class LeastSquares implements Regression
      */
     private $coefficients;
 
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         $this->samples = array_merge($this->samples, $samples);
         $this->targets = array_merge($this->targets, $targets);
@@ -64,7 +64,7 @@ class LeastSquares implements Regression
     /**
      * coefficient(b) = (X'X)-1X'Y.
      */
-    private function computeCoefficients()
+    private function computeCoefficients(): void
     {
         $samplesMatrix = $this->getSamplesMatrix();
         $targetsMatrix = $this->getTargetsMatrix();
diff --git a/src/Phpml/Regression/SVR.php b/src/Phpml/Regression/SVR.php
index 54215e0..e3032ac 100644
--- a/src/Phpml/Regression/SVR.php
+++ b/src/Phpml/Regression/SVR.php
@@ -15,7 +15,7 @@ class SVR extends SupportVectorMachine implements Regression
         int $degree = 3,
         float $epsilon = 0.1,
         float $cost = 1.0,
-        float $gamma = null,
+        ?float $gamma = null,
         float $coef0 = 0.0,
         float $tolerance = 0.001,
         int $cacheSize = 100,
diff --git a/src/Phpml/SupportVectorMachine/Kernel.php b/src/Phpml/SupportVectorMachine/Kernel.php
index 9918a3f..af76e6d 100644
--- a/src/Phpml/SupportVectorMachine/Kernel.php
+++ b/src/Phpml/SupportVectorMachine/Kernel.php
@@ -9,20 +9,20 @@ abstract class Kernel
     /**
      * u'*v.
      */
-    const LINEAR = 0;
+    public const LINEAR = 0;
 
     /**
      * (gamma*u'*v + coef0)^degree.
      */
-    const POLYNOMIAL = 1;
+    public const POLYNOMIAL = 1;
 
     /**
      * exp(-gamma*|u-v|^2).
      */
-    const RBF = 2;
+    public const RBF = 2;
 
     /**
      * tanh(gamma*u'*v + coef0).
      */
-    const SIGMOID = 3;
+    public const SIGMOID = 3;
 }
diff --git a/src/Phpml/SupportVectorMachine/SupportVectorMachine.php b/src/Phpml/SupportVectorMachine/SupportVectorMachine.php
index 6b6c48d..cbee23d 100644
--- a/src/Phpml/SupportVectorMachine/SupportVectorMachine.php
+++ b/src/Phpml/SupportVectorMachine/SupportVectorMachine.php
@@ -97,7 +97,7 @@ class SupportVectorMachine
         float $cost = 1.0,
         float $nu = 0.5,
         int $degree = 3,
-        float $gamma = null,
+        ?float $gamma = null,
         float $coef0 = 0.0,
         float $epsilon = 0.1,
         float $tolerance = 0.001,
@@ -124,7 +124,7 @@ class SupportVectorMachine
         $this->varPath = $rootPath.'var'.DIRECTORY_SEPARATOR;
     }
 
-    public function setBinPath(string $binPath)
+    public function setBinPath(string $binPath): void
     {
         $this->ensureDirectorySeparator($binPath);
         $this->verifyBinPath($binPath);
@@ -132,7 +132,7 @@ class SupportVectorMachine
         $this->binPath = $binPath;
     }
 
-    public function setVarPath(string $varPath)
+    public function setVarPath(string $varPath): void
     {
         if (!is_writable($varPath)) {
             throw InvalidArgumentException::pathNotWritable($varPath);
@@ -142,7 +142,7 @@ class SupportVectorMachine
         $this->varPath = $varPath;
     }
 
-    public function train(array $samples, array $targets)
+    public function train(array $samples, array $targets): void
     {
         $this->samples = array_merge($this->samples, $samples);
         $this->targets = array_merge($this->targets, $targets);
@@ -234,14 +234,14 @@ class SupportVectorMachine
         );
     }
 
-    private function ensureDirectorySeparator(string &$path)
+    private function ensureDirectorySeparator(string &$path): void
     {
         if (substr($path, -1) !== DIRECTORY_SEPARATOR) {
             $path .= DIRECTORY_SEPARATOR;
         }
     }
 
-    private function verifyBinPath(string $path)
+    private function verifyBinPath(string $path): void
     {
         if (!is_dir($path)) {
             throw InvalidArgumentException::pathNotFound($path);
diff --git a/src/Phpml/SupportVectorMachine/Type.php b/src/Phpml/SupportVectorMachine/Type.php
index 1b454a5..1dea9df 100644
--- a/src/Phpml/SupportVectorMachine/Type.php
+++ b/src/Phpml/SupportVectorMachine/Type.php
@@ -9,25 +9,25 @@ abstract class Type
     /**
      * classification.
      */
-    const C_SVC = 0;
+    public const C_SVC = 0;
 
     /**
      * classification.
      */
-    const NU_SVC = 1;
+    public const NU_SVC = 1;
 
     /**
      * distribution estimation.
      */
-    const ONE_CLASS_SVM = 2;
+    public const ONE_CLASS_SVM = 2;
 
     /**
      * regression.
      */
-    const EPSILON_SVR = 3;
+    public const EPSILON_SVR = 3;
 
     /**
      * regression.
      */
-    const NU_SVR = 4;
+    public const NU_SVR = 4;
 }
diff --git a/tests/Phpml/Association/AprioriTest.php b/tests/Phpml/Association/AprioriTest.php
index c715d6f..3b47483 100644
--- a/tests/Phpml/Association/AprioriTest.php
+++ b/tests/Phpml/Association/AprioriTest.php
@@ -40,7 +40,7 @@ class AprioriTest extends TestCase
         [2, 4],
     ];
 
-    public function testGreek()
+    public function testGreek(): void
     {
         $apriori = new Apriori(0.5, 0.5);
         $apriori->train($this->sampleGreek, []);
@@ -49,14 +49,14 @@ class AprioriTest extends TestCase
         $this->assertEquals('alpha', $apriori->predict([['alpha', 'epsilon'], ['beta', 'theta']])[1][0][0]);
     }
 
-    public function testPowerSet()
+    public function testPowerSet(): void
     {
         $apriori = new Apriori();
 
         $this->assertCount(8, $this->invoke($apriori, 'powerSet', [['a', 'b', 'c']]));
     }
 
-    public function testApriori()
+    public function testApriori(): void
     {
         $apriori = new Apriori(3 / 7);
         $apriori->train($this->sampleBasket, []);
@@ -73,7 +73,7 @@ class AprioriTest extends TestCase
         $this->assertTrue($this->invoke($apriori, 'contains', [$L[2], [3, 4]]));
     }
 
-    public function testGetRules()
+    public function testGetRules(): void
     {
         $apriori = new Apriori(0.4, 0.8);
         $apriori->train($this->sampleChars, []);
@@ -81,21 +81,21 @@ class AprioriTest extends TestCase
         $this->assertCount(19, $apriori->getRules());
     }
 
-    public function testAntecedents()
+    public function testAntecedents(): void
     {
         $apriori = new Apriori();
 
         $this->assertCount(6, $this->invoke($apriori, 'antecedents', [['a', 'b', 'c']]));
     }
 
-    public function testItems()
+    public function testItems(): void
     {
         $apriori = new Apriori();
         $apriori->train($this->sampleGreek, []);
         $this->assertCount(4, $this->invoke($apriori, 'items', []));
     }
 
-    public function testFrequent()
+    public function testFrequent(): void
     {
         $apriori = new Apriori(0.51);
         $apriori->train($this->sampleGreek, []);
@@ -104,7 +104,7 @@ class AprioriTest extends TestCase
         $this->assertCount(2, $this->invoke($apriori, 'frequent', [[['alpha'], ['beta']]]));
     }
 
-    public function testCandidates()
+    public function testCandidates(): void
     {
         $apriori = new Apriori();
         $apriori->train($this->sampleGreek, []);
@@ -115,7 +115,7 @@ class AprioriTest extends TestCase
         $this->assertCount(3, $this->invoke($apriori, 'candidates', [[['alpha'], ['beta'], ['theta']]]));
     }
 
-    public function testConfidence()
+    public function testConfidence(): void
     {
         $apriori = new Apriori();
         $apriori->train($this->sampleGreek, []);
@@ -124,7 +124,7 @@ class AprioriTest extends TestCase
         $this->assertEquals(1, $this->invoke($apriori, 'confidence', [['alpha', 'beta'], ['alpha']]));
     }
 
-    public function testSupport()
+    public function testSupport(): void
     {
         $apriori = new Apriori();
         $apriori->train($this->sampleGreek, []);
@@ -133,7 +133,7 @@ class AprioriTest extends TestCase
         $this->assertEquals(0.5, $this->invoke($apriori, 'support', [['epsilon']]));
     }
 
-    public function testFrequency()
+    public function testFrequency(): void
     {
         $apriori = new Apriori();
         $apriori->train($this->sampleGreek, []);
@@ -142,7 +142,7 @@ class AprioriTest extends TestCase
         $this->assertEquals(2, $this->invoke($apriori, 'frequency', [['epsilon']]));
     }
 
-    public function testContains()
+    public function testContains(): void
     {
         $apriori = new Apriori();
 
@@ -151,7 +151,7 @@ class AprioriTest extends TestCase
         $this->assertFalse($this->invoke($apriori, 'contains', [[['a'], ['b']], ['c']]));
     }
 
-    public function testSubset()
+    public function testSubset(): void
     {
         $apriori = new Apriori();
 
@@ -160,7 +160,7 @@ class AprioriTest extends TestCase
         $this->assertFalse($this->invoke($apriori, 'subset', [['a'], ['a', 'b']]));
     }
 
-    public function testEquals()
+    public function testEquals(): void
     {
         $apriori = new Apriori();
 
@@ -187,7 +187,7 @@ class AprioriTest extends TestCase
         return $method->invokeArgs($object, $params);
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         $classifier = new Apriori(0.5, 0.5);
         $classifier->train($this->sampleGreek, []);
diff --git a/tests/Phpml/Classification/DecisionTree/DecisionTreeLeafTest.php b/tests/Phpml/Classification/DecisionTree/DecisionTreeLeafTest.php
index 854e4e5..626ed64 100644
--- a/tests/Phpml/Classification/DecisionTree/DecisionTreeLeafTest.php
+++ b/tests/Phpml/Classification/DecisionTree/DecisionTreeLeafTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class DecisionTreeLeafTest extends TestCase
 {
-    public function testHTMLOutput()
+    public function testHTMLOutput(): void
     {
         $leaf = new DecisionTreeLeaf();
         $leaf->value = 1;
diff --git a/tests/Phpml/Classification/DecisionTreeTest.php b/tests/Phpml/Classification/DecisionTreeTest.php
index f0ebc04..c7d2d2a 100644
--- a/tests/Phpml/Classification/DecisionTreeTest.php
+++ b/tests/Phpml/Classification/DecisionTreeTest.php
@@ -35,7 +35,7 @@ class DecisionTreeTest extends TestCase
     private function getData($input)
     {
         $targets = array_column($input, 4);
-        array_walk($input, function (&$v) {
+        array_walk($input, function (&$v): void {
             array_splice($v, 4, 1);
         });
 
@@ -44,14 +44,14 @@ class DecisionTreeTest extends TestCase
 
     public function testPredictSingleSample()
     {
-        list($data, $targets) = $this->getData($this->data);
+        [$data, $targets] = $this->getData($this->data);
         $classifier = new DecisionTree(5);
         $classifier->train($data, $targets);
         $this->assertEquals('Dont_play', $classifier->predict(['sunny', 78, 72, 'false']));
         $this->assertEquals('Play', $classifier->predict(['overcast', 60, 60, 'false']));
         $this->assertEquals('Dont_play', $classifier->predict(['rain', 60, 60, 'true']));
 
-        list($data, $targets) = $this->getData($this->extraData);
+        [$data, $targets] = $this->getData($this->extraData);
         $classifier->train($data, $targets);
         $this->assertEquals('Dont_play', $classifier->predict(['scorching', 95, 90, 'true']));
         $this->assertEquals('Play', $classifier->predict(['overcast', 60, 60, 'false']));
@@ -59,9 +59,9 @@ class DecisionTreeTest extends TestCase
         return $classifier;
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
-        list($data, $targets) = $this->getData($this->data);
+        [$data, $targets] = $this->getData($this->data);
         $classifier = new DecisionTree(5);
         $classifier->train($data, $targets);
 
@@ -78,9 +78,9 @@ class DecisionTreeTest extends TestCase
         $this->assertEquals($predicted, $restoredClassifier->predict($testSamples));
     }
 
-    public function testTreeDepth()
+    public function testTreeDepth(): void
     {
-        list($data, $targets) = $this->getData($this->data);
+        [$data, $targets] = $this->getData($this->data);
         $classifier = new DecisionTree(5);
         $classifier->train($data, $targets);
         $this->assertTrue(5 >= $classifier->actualDepth);
diff --git a/tests/Phpml/Classification/Ensemble/AdaBoostTest.php b/tests/Phpml/Classification/Ensemble/AdaBoostTest.php
index 8b8d0db..2cc8090 100644
--- a/tests/Phpml/Classification/Ensemble/AdaBoostTest.php
+++ b/tests/Phpml/Classification/Ensemble/AdaBoostTest.php
@@ -42,7 +42,7 @@ class AdaBoostTest extends TestCase
         return $classifier;
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         // Instantinate new Percetron trained for OR problem
         $samples = [[0, 0], [1, 0], [0, 1], [1, 1]];
diff --git a/tests/Phpml/Classification/Ensemble/BaggingTest.php b/tests/Phpml/Classification/Ensemble/BaggingTest.php
index afc358a..5bca8de 100644
--- a/tests/Phpml/Classification/Ensemble/BaggingTest.php
+++ b/tests/Phpml/Classification/Ensemble/BaggingTest.php
@@ -36,7 +36,7 @@ class BaggingTest extends TestCase
 
     public function testPredictSingleSample()
     {
-        list($data, $targets) = $this->getData($this->data);
+        [$data, $targets] = $this->getData($this->data);
         $classifier = $this->getClassifier();
         // Testing with default options
         $classifier->train($data, $targets);
@@ -44,7 +44,7 @@ class BaggingTest extends TestCase
         $this->assertEquals('Play', $classifier->predict(['overcast', 60, 60, 'false']));
         $this->assertEquals('Dont_play', $classifier->predict(['rain', 60, 60, 'true']));
 
-        list($data, $targets) = $this->getData($this->extraData);
+        [$data, $targets] = $this->getData($this->extraData);
         $classifier->train($data, $targets);
         $this->assertEquals('Dont_play', $classifier->predict(['scorching', 95, 90, 'true']));
         $this->assertEquals('Play', $classifier->predict(['overcast', 60, 60, 'false']));
@@ -52,9 +52,9 @@ class BaggingTest extends TestCase
         return $classifier;
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
-        list($data, $targets) = $this->getData($this->data);
+        [$data, $targets] = $this->getData($this->data);
         $classifier = $this->getClassifier(5);
         $classifier->train($data, $targets);
 
@@ -71,9 +71,9 @@ class BaggingTest extends TestCase
         $this->assertEquals($predicted, $restoredClassifier->predict($testSamples));
     }
 
-    public function testBaseClassifiers()
+    public function testBaseClassifiers(): void
     {
-        list($data, $targets) = $this->getData($this->data);
+        [$data, $targets] = $this->getData($this->data);
         $baseClassifiers = $this->getAvailableBaseClassifiers();
 
         foreach ($baseClassifiers as $base => $params) {
@@ -119,7 +119,7 @@ class BaggingTest extends TestCase
         }
         shuffle($populated);
         $targets = array_column($populated, 4);
-        array_walk($populated, function (&$v) {
+        array_walk($populated, function (&$v): void {
             array_splice($v, 4, 1);
         });
 
diff --git a/tests/Phpml/Classification/Ensemble/RandomForestTest.php b/tests/Phpml/Classification/Ensemble/RandomForestTest.php
index 3ff51e0..e4ca9e5 100644
--- a/tests/Phpml/Classification/Ensemble/RandomForestTest.php
+++ b/tests/Phpml/Classification/Ensemble/RandomForestTest.php
@@ -23,7 +23,7 @@ class RandomForestTest extends BaggingTest
         return [DecisionTree::class => ['depth' => 5]];
     }
 
-    public function testOtherBaseClassifier()
+    public function testOtherBaseClassifier(): void
     {
         try {
             $classifier = new RandomForest();
diff --git a/tests/Phpml/Classification/KNearestNeighborsTest.php b/tests/Phpml/Classification/KNearestNeighborsTest.php
index aeee5ef..7ef6182 100644
--- a/tests/Phpml/Classification/KNearestNeighborsTest.php
+++ b/tests/Phpml/Classification/KNearestNeighborsTest.php
@@ -11,7 +11,7 @@ use PHPUnit\Framework\TestCase;
 
 class KNearestNeighborsTest extends TestCase
 {
-    public function testPredictSingleSampleWithDefaultK()
+    public function testPredictSingleSampleWithDefaultK(): void
     {
         $samples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
         $labels = ['a', 'a', 'a', 'b', 'b', 'b'];
@@ -30,7 +30,7 @@ class KNearestNeighborsTest extends TestCase
         $this->assertEquals('a', $classifier->predict([3, 10]));
     }
 
-    public function testPredictArrayOfSamples()
+    public function testPredictArrayOfSamples(): void
     {
         $trainSamples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
         $trainLabels = ['a', 'a', 'a', 'b', 'b', 'b'];
@@ -45,7 +45,7 @@ class KNearestNeighborsTest extends TestCase
         $this->assertEquals($testLabels, $predicted);
     }
 
-    public function testPredictArrayOfSamplesUsingChebyshevDistanceMetric()
+    public function testPredictArrayOfSamplesUsingChebyshevDistanceMetric(): void
     {
         $trainSamples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
         $trainLabels = ['a', 'a', 'a', 'b', 'b', 'b'];
@@ -60,7 +60,7 @@ class KNearestNeighborsTest extends TestCase
         $this->assertEquals($testLabels, $predicted);
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         $trainSamples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
         $trainLabels = ['a', 'a', 'a', 'b', 'b', 'b'];
diff --git a/tests/Phpml/Classification/Linear/AdalineTest.php b/tests/Phpml/Classification/Linear/AdalineTest.php
index 5989462..46f1a34 100644
--- a/tests/Phpml/Classification/Linear/AdalineTest.php
+++ b/tests/Phpml/Classification/Linear/AdalineTest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class AdalineTest extends TestCase
 {
-    public function testPredictSingleSample()
+    public function testPredictSingleSample(): void
     {
         // AND problem
         $samples = [[0, 0], [1, 0], [0, 1], [1, 1]];
@@ -64,7 +64,7 @@ class AdalineTest extends TestCase
         $this->assertEquals(1, $classifier->predict([3.0, 9.5]));
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         // Instantinate new Percetron trained for OR problem
         $samples = [[0, 0], [1, 0], [0, 1], [1, 1]];
diff --git a/tests/Phpml/Classification/Linear/DecisionStumpTest.php b/tests/Phpml/Classification/Linear/DecisionStumpTest.php
index 678544e..fa522ad 100644
--- a/tests/Phpml/Classification/Linear/DecisionStumpTest.php
+++ b/tests/Phpml/Classification/Linear/DecisionStumpTest.php
@@ -53,7 +53,7 @@ class DecisionStumpTest extends TestCase
         return $classifier;
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         // Instantinate new Percetron trained for OR problem
         $samples = [[0, 0], [1, 0], [0, 1], [1, 1]];
diff --git a/tests/Phpml/Classification/Linear/PerceptronTest.php b/tests/Phpml/Classification/Linear/PerceptronTest.php
index c01da33..2721067 100644
--- a/tests/Phpml/Classification/Linear/PerceptronTest.php
+++ b/tests/Phpml/Classification/Linear/PerceptronTest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class PerceptronTest extends TestCase
 {
-    public function testPredictSingleSample()
+    public function testPredictSingleSample(): void
     {
         // AND problem
         $samples = [[0, 0], [1, 0], [0, 1], [1, 1], [0.6, 0.6]];
@@ -67,7 +67,7 @@ class PerceptronTest extends TestCase
         $this->assertEquals(1, $classifier->predict([3.0, 9.5]));
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         // Instantinate new Percetron trained for OR problem
         $samples = [[0, 0], [1, 0], [0, 1], [1, 1]];
diff --git a/tests/Phpml/Classification/MLPClassifierTest.php b/tests/Phpml/Classification/MLPClassifierTest.php
index 1745bc6..9f9c9a9 100644
--- a/tests/Phpml/Classification/MLPClassifierTest.php
+++ b/tests/Phpml/Classification/MLPClassifierTest.php
@@ -11,7 +11,7 @@ use PHPUnit\Framework\TestCase;
 
 class MLPClassifierTest extends TestCase
 {
-    public function testMLPClassifierLayersInitialization()
+    public function testMLPClassifierLayersInitialization(): void
     {
         $mlp = new MLPClassifier(2, [2], [0, 1]);
 
@@ -32,7 +32,7 @@ class MLPClassifierTest extends TestCase
         $this->assertContainsOnly(Neuron::class, $layers[2]->getNodes());
     }
 
-    public function testSynapsesGeneration()
+    public function testSynapsesGeneration(): void
     {
         $mlp = new MLPClassifier(2, [2], [0, 1]);
         $layers = $mlp->getLayers();
@@ -50,7 +50,7 @@ class MLPClassifierTest extends TestCase
         }
     }
 
-    public function testBackpropagationLearning()
+    public function testBackpropagationLearning(): void
     {
         // Single layer 2 classes.
         $network = new MLPClassifier(2, [2], ['a', 'b']);
@@ -65,7 +65,7 @@ class MLPClassifierTest extends TestCase
         $this->assertEquals('b', $network->predict([0, 0]));
     }
 
-    public function testBackpropagationTrainingReset()
+    public function testBackpropagationTrainingReset(): void
     {
         // Single layer 2 classes.
         $network = new MLPClassifier(2, [2], ['a', 'b'], 1000);
@@ -86,7 +86,7 @@ class MLPClassifierTest extends TestCase
         $this->assertEquals('a', $network->predict([0, 1]));
     }
 
-    public function testBackpropagationPartialTraining()
+    public function testBackpropagationPartialTraining(): void
     {
         // Single layer 2 classes.
         $network = new MLPClassifier(2, [2], ['a', 'b'], 1000);
@@ -109,7 +109,7 @@ class MLPClassifierTest extends TestCase
         $this->assertEquals('b', $network->predict([0, 0]));
     }
 
-    public function testBackpropagationLearningMultilayer()
+    public function testBackpropagationLearningMultilayer(): void
     {
         // Multi-layer 2 classes.
         $network = new MLPClassifier(5, [3, 2], ['a', 'b']);
@@ -124,7 +124,7 @@ class MLPClassifierTest extends TestCase
         $this->assertEquals('b', $network->predict([0, 0, 0, 0, 0]));
     }
 
-    public function testBackpropagationLearningMulticlass()
+    public function testBackpropagationLearningMulticlass(): void
     {
         // Multi-layer more than 2 classes.
         $network = new MLPClassifier(5, [3, 2], ['a', 'b', 4]);
@@ -140,7 +140,7 @@ class MLPClassifierTest extends TestCase
         $this->assertEquals(4, $network->predict([0, 0, 0, 0, 0]));
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         // Instantinate new Percetron trained for OR problem
         $samples = [[0, 0], [1, 0], [0, 1], [1, 1]];
@@ -163,7 +163,7 @@ class MLPClassifierTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidLayersNumber()
+    public function testThrowExceptionOnInvalidLayersNumber(): void
     {
         new MLPClassifier(2, [], [0, 1]);
     }
@@ -171,7 +171,7 @@ class MLPClassifierTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidPartialTrainingClasses()
+    public function testThrowExceptionOnInvalidPartialTrainingClasses(): void
     {
         $classifier = new MLPClassifier(2, [2], [0, 1]);
         $classifier->partialTrain(
@@ -184,7 +184,7 @@ class MLPClassifierTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidClassesNumber()
+    public function testThrowExceptionOnInvalidClassesNumber(): void
     {
         new MLPClassifier(2, [2], [0]);
     }
diff --git a/tests/Phpml/Classification/NaiveBayesTest.php b/tests/Phpml/Classification/NaiveBayesTest.php
index 82c69eb..5b14f7e 100644
--- a/tests/Phpml/Classification/NaiveBayesTest.php
+++ b/tests/Phpml/Classification/NaiveBayesTest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class NaiveBayesTest extends TestCase
 {
-    public function testPredictSingleSample()
+    public function testPredictSingleSample(): void
     {
         $samples = [[5, 1, 1], [1, 5, 1], [1, 1, 5]];
         $labels = ['a', 'b', 'c'];
@@ -23,7 +23,7 @@ class NaiveBayesTest extends TestCase
         $this->assertEquals('c', $classifier->predict([1, 1, 6]));
     }
 
-    public function testPredictArrayOfSamples()
+    public function testPredictArrayOfSamples(): void
     {
         $trainSamples = [[5, 1, 1], [1, 5, 1], [1, 1, 5]];
         $trainLabels = ['a', 'b', 'c'];
@@ -47,7 +47,7 @@ class NaiveBayesTest extends TestCase
         $this->assertEquals($testLabels, $classifier->predict($testSamples));
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         $trainSamples = [[5, 1, 1], [1, 5, 1], [1, 1, 5]];
         $trainLabels = ['a', 'b', 'c'];
diff --git a/tests/Phpml/Classification/SVCTest.php b/tests/Phpml/Classification/SVCTest.php
index 1b529d0..f734297 100644
--- a/tests/Phpml/Classification/SVCTest.php
+++ b/tests/Phpml/Classification/SVCTest.php
@@ -11,7 +11,7 @@ use PHPUnit\Framework\TestCase;
 
 class SVCTest extends TestCase
 {
-    public function testPredictSingleSampleWithLinearKernel()
+    public function testPredictSingleSampleWithLinearKernel(): void
     {
         $samples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
         $labels = ['a', 'a', 'a', 'b', 'b', 'b'];
@@ -30,7 +30,7 @@ class SVCTest extends TestCase
         $this->assertEquals('a', $classifier->predict([3, 10]));
     }
 
-    public function testPredictArrayOfSamplesWithLinearKernel()
+    public function testPredictArrayOfSamplesWithLinearKernel(): void
     {
         $trainSamples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
         $trainLabels = ['a', 'a', 'a', 'b', 'b', 'b'];
@@ -45,7 +45,7 @@ class SVCTest extends TestCase
         $this->assertEquals($testLabels, $predictions);
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         $trainSamples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
         $trainLabels = ['a', 'a', 'a', 'b', 'b', 'b'];
diff --git a/tests/Phpml/Clustering/DBSCANTest.php b/tests/Phpml/Clustering/DBSCANTest.php
index a093b20..d8fb0fe 100644
--- a/tests/Phpml/Clustering/DBSCANTest.php
+++ b/tests/Phpml/Clustering/DBSCANTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class DBSCANTest extends TestCase
 {
-    public function testDBSCANSamplesClustering()
+    public function testDBSCANSamplesClustering(): void
     {
         $samples = [[1, 1], [8, 7], [1, 2], [7, 8], [2, 1], [8, 9]];
         $clustered = [
@@ -32,7 +32,7 @@ class DBSCANTest extends TestCase
         $this->assertEquals($clustered, $dbscan->cluster($samples));
     }
 
-    public function testDBSCANSamplesClusteringAssociative()
+    public function testDBSCANSamplesClusteringAssociative(): void
     {
         $samples = ['a' => [1, 1], 'b' => [9, 9], 'c' => [1, 2], 'd' => [9, 8], 'e' => [7, 7], 'f' => [8, 7]];
         $clustered = [
diff --git a/tests/Phpml/Clustering/FuzzyCMeansTest.php b/tests/Phpml/Clustering/FuzzyCMeansTest.php
index 39f2643..5aed678 100644
--- a/tests/Phpml/Clustering/FuzzyCMeansTest.php
+++ b/tests/Phpml/Clustering/FuzzyCMeansTest.php
@@ -25,7 +25,7 @@ class FuzzyCMeansTest extends TestCase
         return $fcm;
     }
 
-    public function testMembershipMatrix()
+    public function testMembershipMatrix(): void
     {
         $fcm = $this->testFCMSamplesClustering();
         $clusterCount = 2;
diff --git a/tests/Phpml/Clustering/KMeansTest.php b/tests/Phpml/Clustering/KMeansTest.php
index 43d41ee..e665c9f 100644
--- a/tests/Phpml/Clustering/KMeansTest.php
+++ b/tests/Phpml/Clustering/KMeansTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class KMeansTest extends TestCase
 {
-    public function testKMeansSamplesClustering()
+    public function testKMeansSamplesClustering(): void
     {
         $samples = [[1, 1], [8, 7], [1, 2], [7, 8], [2, 1], [8, 9]];
 
@@ -26,7 +26,7 @@ class KMeansTest extends TestCase
         $this->assertCount(0, $samples);
     }
 
-    public function testKMeansInitializationMethods()
+    public function testKMeansInitializationMethods(): void
     {
         $samples = [
             [180, 155], [186, 159], [119, 185], [141, 147], [157, 158],
@@ -53,7 +53,7 @@ class KMeansTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidClusterNumber()
+    public function testThrowExceptionOnInvalidClusterNumber(): void
     {
         new KMeans(0);
     }
diff --git a/tests/Phpml/CrossValidation/RandomSplitTest.php b/tests/Phpml/CrossValidation/RandomSplitTest.php
index b2717fd..070e36b 100644
--- a/tests/Phpml/CrossValidation/RandomSplitTest.php
+++ b/tests/Phpml/CrossValidation/RandomSplitTest.php
@@ -13,7 +13,7 @@ class RandomSplitTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnToSmallTestSize()
+    public function testThrowExceptionOnToSmallTestSize(): void
     {
         new RandomSplit(new ArrayDataset([], []), 0);
     }
@@ -21,12 +21,12 @@ class RandomSplitTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnToBigTestSize()
+    public function testThrowExceptionOnToBigTestSize(): void
     {
         new RandomSplit(new ArrayDataset([], []), 1);
     }
 
-    public function testDatasetRandomSplitWithoutSeed()
+    public function testDatasetRandomSplitWithoutSeed(): void
     {
         $dataset = new ArrayDataset(
             $samples = [[1], [2], [3], [4]],
@@ -44,7 +44,7 @@ class RandomSplitTest extends TestCase
         $this->assertCount(3, $randomSplit2->getTrainSamples());
     }
 
-    public function testDatasetRandomSplitWithSameSeed()
+    public function testDatasetRandomSplitWithSameSeed(): void
     {
         $dataset = new ArrayDataset(
             $samples = [[1], [2], [3], [4], [5], [6], [7], [8]],
@@ -62,7 +62,7 @@ class RandomSplitTest extends TestCase
         $this->assertEquals($randomSplit1->getTrainSamples(), $randomSplit2->getTrainSamples());
     }
 
-    public function testDatasetRandomSplitWithDifferentSeed()
+    public function testDatasetRandomSplitWithDifferentSeed(): void
     {
         $dataset = new ArrayDataset(
             $samples = [[1], [2], [3], [4], [5], [6], [7], [8]],
@@ -78,7 +78,7 @@ class RandomSplitTest extends TestCase
         $this->assertNotEquals($randomSplit1->getTrainSamples(), $randomSplit2->getTrainSamples());
     }
 
-    public function testRandomSplitCorrectSampleAndLabelPosition()
+    public function testRandomSplitCorrectSampleAndLabelPosition(): void
     {
         $dataset = new ArrayDataset(
             $samples = [[1], [2], [3], [4]],
diff --git a/tests/Phpml/CrossValidation/StratifiedRandomSplitTest.php b/tests/Phpml/CrossValidation/StratifiedRandomSplitTest.php
index 57023d3..e953ca7 100644
--- a/tests/Phpml/CrossValidation/StratifiedRandomSplitTest.php
+++ b/tests/Phpml/CrossValidation/StratifiedRandomSplitTest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class StratifiedRandomSplitTest extends TestCase
 {
-    public function testDatasetStratifiedRandomSplitWithEvenDistribution()
+    public function testDatasetStratifiedRandomSplitWithEvenDistribution(): void
     {
         $dataset = new ArrayDataset(
             $samples = [[1], [2], [3], [4], [5], [6], [7], [8]],
@@ -28,7 +28,7 @@ class StratifiedRandomSplitTest extends TestCase
         $this->assertEquals(1, $this->countSamplesByTarget($split->getTestLabels(), 'b'));
     }
 
-    public function testDatasetStratifiedRandomSplitWithEvenDistributionAndNumericTargets()
+    public function testDatasetStratifiedRandomSplitWithEvenDistributionAndNumericTargets(): void
     {
         $dataset = new ArrayDataset(
             $samples = [[1], [2], [3], [4], [5], [6], [7], [8]],
diff --git a/tests/Phpml/Dataset/ArrayDatasetTest.php b/tests/Phpml/Dataset/ArrayDatasetTest.php
index 0b13b51..41e037b 100644
--- a/tests/Phpml/Dataset/ArrayDatasetTest.php
+++ b/tests/Phpml/Dataset/ArrayDatasetTest.php
@@ -12,12 +12,12 @@ class ArrayDatasetTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidArgumentsSize()
+    public function testThrowExceptionOnInvalidArgumentsSize(): void
     {
         new ArrayDataset([0, 1], [0]);
     }
 
-    public function testArrayDataset()
+    public function testArrayDataset(): void
     {
         $dataset = new ArrayDataset(
             $samples = [[1], [2], [3], [4]],
diff --git a/tests/Phpml/Dataset/CsvDatasetTest.php b/tests/Phpml/Dataset/CsvDatasetTest.php
index a3a377e..f5cc851 100644
--- a/tests/Phpml/Dataset/CsvDatasetTest.php
+++ b/tests/Phpml/Dataset/CsvDatasetTest.php
@@ -12,12 +12,12 @@ class CsvDatasetTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\FileException
      */
-    public function testThrowExceptionOnMissingFile()
+    public function testThrowExceptionOnMissingFile(): void
     {
         new CsvDataset('missingFile', 3);
     }
 
-    public function testSampleCsvDatasetWithHeaderRow()
+    public function testSampleCsvDatasetWithHeaderRow(): void
     {
         $filePath = dirname(__FILE__).'/Resources/dataset.csv';
 
@@ -27,7 +27,7 @@ class CsvDatasetTest extends TestCase
         $this->assertCount(10, $dataset->getTargets());
     }
 
-    public function testSampleCsvDatasetWithoutHeaderRow()
+    public function testSampleCsvDatasetWithoutHeaderRow(): void
     {
         $filePath = dirname(__FILE__).'/Resources/dataset.csv';
 
@@ -37,7 +37,7 @@ class CsvDatasetTest extends TestCase
         $this->assertCount(11, $dataset->getTargets());
     }
 
-    public function testLongCsvDataset()
+    public function testLongCsvDataset(): void
     {
         $filePath = dirname(__FILE__).'/Resources/longdataset.csv';
 
diff --git a/tests/Phpml/Dataset/Demo/GlassDatasetTest.php b/tests/Phpml/Dataset/Demo/GlassDatasetTest.php
index 5e3f94c..8feef45 100644
--- a/tests/Phpml/Dataset/Demo/GlassDatasetTest.php
+++ b/tests/Phpml/Dataset/Demo/GlassDatasetTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class GlassDatasetTest extends TestCase
 {
-    public function testLoadingWineDataset()
+    public function testLoadingWineDataset(): void
     {
         $glass = new GlassDataset();
 
diff --git a/tests/Phpml/Dataset/Demo/IrisDatasetTest.php b/tests/Phpml/Dataset/Demo/IrisDatasetTest.php
index 18ad647..faa48c6 100644
--- a/tests/Phpml/Dataset/Demo/IrisDatasetTest.php
+++ b/tests/Phpml/Dataset/Demo/IrisDatasetTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class IrisDatasetTest extends TestCase
 {
-    public function testLoadingIrisDataset()
+    public function testLoadingIrisDataset(): void
     {
         $iris = new IrisDataset();
 
diff --git a/tests/Phpml/Dataset/Demo/WineDatasetTest.php b/tests/Phpml/Dataset/Demo/WineDatasetTest.php
index a79ed8c..e0324b4 100644
--- a/tests/Phpml/Dataset/Demo/WineDatasetTest.php
+++ b/tests/Phpml/Dataset/Demo/WineDatasetTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class WineDatasetTest extends TestCase
 {
-    public function testLoadingWineDataset()
+    public function testLoadingWineDataset(): void
     {
         $wine = new WineDataset();
 
diff --git a/tests/Phpml/Dataset/FilesDatasetTest.php b/tests/Phpml/Dataset/FilesDatasetTest.php
index d12477f..0592d06 100644
--- a/tests/Phpml/Dataset/FilesDatasetTest.php
+++ b/tests/Phpml/Dataset/FilesDatasetTest.php
@@ -12,12 +12,12 @@ class FilesDatasetTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\DatasetException
      */
-    public function testThrowExceptionOnMissingRootFolder()
+    public function testThrowExceptionOnMissingRootFolder(): void
     {
         new FilesDataset('some/not/existed/path');
     }
 
-    public function testLoadFilesDatasetWithBBCData()
+    public function testLoadFilesDatasetWithBBCData(): void
     {
         $rootPath = dirname(__FILE__).'/Resources/bbc';
 
diff --git a/tests/Phpml/DimensionReduction/KernelPCATest.php b/tests/Phpml/DimensionReduction/KernelPCATest.php
index e586fc8..66b4622 100644
--- a/tests/Phpml/DimensionReduction/KernelPCATest.php
+++ b/tests/Phpml/DimensionReduction/KernelPCATest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class KernelPCATest extends TestCase
 {
-    public function testKernelPCA()
+    public function testKernelPCA(): void
     {
         // Acceptable error
         $epsilon = 0.001;
@@ -37,7 +37,7 @@ class KernelPCATest extends TestCase
         // Due to the fact that the sign of values can be flipped
         // during the calculation of eigenValues, we have to compare
         // absolute value of the values
-        array_map(function ($val1, $val2) use ($epsilon) {
+        array_map(function ($val1, $val2) use ($epsilon): void {
             $this->assertEquals(abs($val1), abs($val2), '', $epsilon);
         }, $transformed, $reducedData);
 
diff --git a/tests/Phpml/DimensionReduction/LDATest.php b/tests/Phpml/DimensionReduction/LDATest.php
index 4498ea5..19124a0 100644
--- a/tests/Phpml/DimensionReduction/LDATest.php
+++ b/tests/Phpml/DimensionReduction/LDATest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class LDATest extends TestCase
 {
-    public function testLDA()
+    public function testLDA(): void
     {
         // Acceptable error
         $epsilon = 0.001;
@@ -43,7 +43,7 @@ class LDATest extends TestCase
         $control = array_merge($control, array_slice($transformed, 0, 3));
         $control = array_merge($control, array_slice($transformed, -3));
 
-        $check = function ($row1, $row2) use ($epsilon) {
+        $check = function ($row1, $row2) use ($epsilon): void {
             // Due to the fact that the sign of values can be flipped
             // during the calculation of eigenValues, we have to compare
             // absolute value of the values
diff --git a/tests/Phpml/DimensionReduction/PCATest.php b/tests/Phpml/DimensionReduction/PCATest.php
index c26782a..0a60004 100644
--- a/tests/Phpml/DimensionReduction/PCATest.php
+++ b/tests/Phpml/DimensionReduction/PCATest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class PCATest extends TestCase
 {
-    public function testPCA()
+    public function testPCA(): void
     {
         // Acceptable error
         $epsilon = 0.001;
@@ -39,7 +39,7 @@ class PCATest extends TestCase
         // Due to the fact that the sign of values can be flipped
         // during the calculation of eigenValues, we have to compare
         // absolute value of the values
-        array_map(function ($val1, $val2) use ($epsilon) {
+        array_map(function ($val1, $val2) use ($epsilon): void {
             $this->assertEquals(abs($val1), abs($val2), '', $epsilon);
         }, $transformed, $reducedData);
 
@@ -49,7 +49,7 @@ class PCATest extends TestCase
             $newRow = [[$transformed[$i]]];
             $newRow2 = $pca->transform($row);
 
-            array_map(function ($val1, $val2) use ($epsilon) {
+            array_map(function ($val1, $val2) use ($epsilon): void {
                 $this->assertEquals(abs($val1), abs($val2), '', $epsilon);
             }, $newRow, $newRow2);
         }
diff --git a/tests/Phpml/FeatureExtraction/StopWordsTest.php b/tests/Phpml/FeatureExtraction/StopWordsTest.php
index 4979860..4b715ef 100644
--- a/tests/Phpml/FeatureExtraction/StopWordsTest.php
+++ b/tests/Phpml/FeatureExtraction/StopWordsTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class StopWordsTest extends TestCase
 {
-    public function testCustomStopWords()
+    public function testCustomStopWords(): void
     {
         $stopWords = new StopWords(['lorem', 'ipsum', 'dolor']);
 
@@ -25,12 +25,12 @@ class StopWordsTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidLanguage()
+    public function testThrowExceptionOnInvalidLanguage(): void
     {
         StopWords::factory('Lorem');
     }
 
-    public function testEnglishStopWords()
+    public function testEnglishStopWords(): void
     {
         $stopWords = StopWords::factory('English');
 
@@ -38,7 +38,7 @@ class StopWordsTest extends TestCase
         $this->assertFalse($stopWords->isStopWord('strategy'));
     }
 
-    public function testPolishStopWords()
+    public function testPolishStopWords(): void
     {
         $stopWords = StopWords::factory('Polish');
 
@@ -46,7 +46,7 @@ class StopWordsTest extends TestCase
         $this->assertFalse($stopWords->isStopWord('transhumanizm'));
     }
 
-    public function testFrenchStopWords()
+    public function testFrenchStopWords(): void
     {
         $stopWords = StopWords::factory('French');
 
diff --git a/tests/Phpml/FeatureExtraction/TfIdfTransformerTest.php b/tests/Phpml/FeatureExtraction/TfIdfTransformerTest.php
index 90aa107..0e1adbc 100644
--- a/tests/Phpml/FeatureExtraction/TfIdfTransformerTest.php
+++ b/tests/Phpml/FeatureExtraction/TfIdfTransformerTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class TfIdfTransformerTest extends TestCase
 {
-    public function testTfIdfTransformation()
+    public function testTfIdfTransformation(): void
     {
         // https://en.wikipedia.org/wiki/Tf-idf
 
diff --git a/tests/Phpml/FeatureExtraction/TokenCountVectorizerTest.php b/tests/Phpml/FeatureExtraction/TokenCountVectorizerTest.php
index a25d5a5..3419a29 100644
--- a/tests/Phpml/FeatureExtraction/TokenCountVectorizerTest.php
+++ b/tests/Phpml/FeatureExtraction/TokenCountVectorizerTest.php
@@ -11,7 +11,7 @@ use PHPUnit\Framework\TestCase;
 
 class TokenCountVectorizerTest extends TestCase
 {
-    public function testTransformationWithWhitespaceTokenizer()
+    public function testTransformationWithWhitespaceTokenizer(): void
     {
         $samples = [
             'Lorem ipsum dolor sit amet dolor',
@@ -47,7 +47,7 @@ class TokenCountVectorizerTest extends TestCase
         $this->assertSame($tokensCounts, $samples);
     }
 
-    public function testTransformationWithMinimumDocumentTokenCountFrequency()
+    public function testTransformationWithMinimumDocumentTokenCountFrequency(): void
     {
         // word at least in half samples
         $samples = [
@@ -100,7 +100,7 @@ class TokenCountVectorizerTest extends TestCase
         $this->assertSame($tokensCounts, $samples);
     }
 
-    public function testTransformationWithStopWords()
+    public function testTransformationWithStopWords(): void
     {
         $samples = [
             'Lorem ipsum dolor sit amet dolor',
diff --git a/tests/Phpml/Math/ComparisonTest.php b/tests/Phpml/Math/ComparisonTest.php
index 8fff3cb..d31b1ae 100644
--- a/tests/Phpml/Math/ComparisonTest.php
+++ b/tests/Phpml/Math/ComparisonTest.php
@@ -15,7 +15,7 @@ class ComparisonTest extends TestCase
      *
      * @dataProvider provideData
      */
-    public function testResult($a, $b, string $operator, bool $expected)
+    public function testResult($a, $b, string $operator, bool $expected): void
     {
         $result = Comparison::compare($a, $b, $operator);
 
@@ -26,7 +26,7 @@ class ComparisonTest extends TestCase
     * @expectedException \Phpml\Exception\InvalidArgumentException
     * @expectedExceptionMessage Invalid operator "~=" provided
     */
-    public function testThrowExceptionWhenOperatorIsInvalid()
+    public function testThrowExceptionWhenOperatorIsInvalid(): void
     {
         Comparison::compare(1, 1, '~=');
     }
diff --git a/tests/Phpml/Math/Distance/ChebyshevTest.php b/tests/Phpml/Math/Distance/ChebyshevTest.php
index 8806c74..893c000 100644
--- a/tests/Phpml/Math/Distance/ChebyshevTest.php
+++ b/tests/Phpml/Math/Distance/ChebyshevTest.php
@@ -14,7 +14,7 @@ class ChebyshevTest extends TestCase
      */
     private $distanceMetric;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->distanceMetric = new Chebyshev();
     }
@@ -22,7 +22,7 @@ class ChebyshevTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidArguments()
+    public function testThrowExceptionOnInvalidArguments(): void
     {
         $a = [0, 1, 2];
         $b = [0, 2];
@@ -30,7 +30,7 @@ class ChebyshevTest extends TestCase
         $this->distanceMetric->distance($a, $b);
     }
 
-    public function testCalculateDistanceForOneDimension()
+    public function testCalculateDistanceForOneDimension(): void
     {
         $a = [4];
         $b = [2];
@@ -41,7 +41,7 @@ class ChebyshevTest extends TestCase
         $this->assertEquals($expectedDistance, $actualDistance);
     }
 
-    public function testCalculateDistanceForTwoDimensions()
+    public function testCalculateDistanceForTwoDimensions(): void
     {
         $a = [4, 6];
         $b = [2, 5];
@@ -52,7 +52,7 @@ class ChebyshevTest extends TestCase
         $this->assertEquals($expectedDistance, $actualDistance);
     }
 
-    public function testCalculateDistanceForThreeDimensions()
+    public function testCalculateDistanceForThreeDimensions(): void
     {
         $a = [6, 10, 3];
         $b = [2, 5, 5];
diff --git a/tests/Phpml/Math/Distance/EuclideanTest.php b/tests/Phpml/Math/Distance/EuclideanTest.php
index 38cb6b7..03bf7f3 100644
--- a/tests/Phpml/Math/Distance/EuclideanTest.php
+++ b/tests/Phpml/Math/Distance/EuclideanTest.php
@@ -14,7 +14,7 @@ class EuclideanTest extends TestCase
      */
     private $distanceMetric;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->distanceMetric = new Euclidean();
     }
@@ -22,7 +22,7 @@ class EuclideanTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidArguments()
+    public function testThrowExceptionOnInvalidArguments(): void
     {
         $a = [0, 1, 2];
         $b = [0, 2];
@@ -30,7 +30,7 @@ class EuclideanTest extends TestCase
         $this->distanceMetric->distance($a, $b);
     }
 
-    public function testCalculateDistanceForOneDimension()
+    public function testCalculateDistanceForOneDimension(): void
     {
         $a = [4];
         $b = [2];
@@ -41,7 +41,7 @@ class EuclideanTest extends TestCase
         $this->assertEquals($expectedDistance, $actualDistance);
     }
 
-    public function testCalculateDistanceForTwoDimensions()
+    public function testCalculateDistanceForTwoDimensions(): void
     {
         $a = [4, 6];
         $b = [2, 5];
@@ -52,7 +52,7 @@ class EuclideanTest extends TestCase
         $this->assertEquals($expectedDistance, $actualDistance);
     }
 
-    public function testCalculateDistanceForThreeDimensions()
+    public function testCalculateDistanceForThreeDimensions(): void
     {
         $a = [6, 10, 3];
         $b = [2, 5, 5];
diff --git a/tests/Phpml/Math/Distance/ManhattanTest.php b/tests/Phpml/Math/Distance/ManhattanTest.php
index 0e22a15..9c20edd 100644
--- a/tests/Phpml/Math/Distance/ManhattanTest.php
+++ b/tests/Phpml/Math/Distance/ManhattanTest.php
@@ -14,7 +14,7 @@ class ManhattanTest extends TestCase
      */
     private $distanceMetric;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->distanceMetric = new Manhattan();
     }
@@ -22,7 +22,7 @@ class ManhattanTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidArguments()
+    public function testThrowExceptionOnInvalidArguments(): void
     {
         $a = [0, 1, 2];
         $b = [0, 2];
@@ -30,7 +30,7 @@ class ManhattanTest extends TestCase
         $this->distanceMetric->distance($a, $b);
     }
 
-    public function testCalculateDistanceForOneDimension()
+    public function testCalculateDistanceForOneDimension(): void
     {
         $a = [4];
         $b = [2];
@@ -41,7 +41,7 @@ class ManhattanTest extends TestCase
         $this->assertEquals($expectedDistance, $actualDistance);
     }
 
-    public function testCalculateDistanceForTwoDimensions()
+    public function testCalculateDistanceForTwoDimensions(): void
     {
         $a = [4, 6];
         $b = [2, 5];
@@ -52,7 +52,7 @@ class ManhattanTest extends TestCase
         $this->assertEquals($expectedDistance, $actualDistance);
     }
 
-    public function testCalculateDistanceForThreeDimensions()
+    public function testCalculateDistanceForThreeDimensions(): void
     {
         $a = [6, 10, 3];
         $b = [2, 5, 5];
diff --git a/tests/Phpml/Math/Distance/MinkowskiTest.php b/tests/Phpml/Math/Distance/MinkowskiTest.php
index d8931d0..81ecd97 100644
--- a/tests/Phpml/Math/Distance/MinkowskiTest.php
+++ b/tests/Phpml/Math/Distance/MinkowskiTest.php
@@ -14,7 +14,7 @@ class MinkowskiTest extends TestCase
      */
     private $distanceMetric;
 
-    public function setUp()
+    public function setUp(): void
     {
         $this->distanceMetric = new Minkowski();
     }
@@ -22,7 +22,7 @@ class MinkowskiTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidArguments()
+    public function testThrowExceptionOnInvalidArguments(): void
     {
         $a = [0, 1, 2];
         $b = [0, 2];
@@ -30,7 +30,7 @@ class MinkowskiTest extends TestCase
         $this->distanceMetric->distance($a, $b);
     }
 
-    public function testCalculateDistanceForOneDimension()
+    public function testCalculateDistanceForOneDimension(): void
     {
         $a = [4];
         $b = [2];
@@ -41,7 +41,7 @@ class MinkowskiTest extends TestCase
         $this->assertEquals($expectedDistance, $actualDistance);
     }
 
-    public function testCalculateDistanceForTwoDimensions()
+    public function testCalculateDistanceForTwoDimensions(): void
     {
         $a = [4, 6];
         $b = [2, 5];
@@ -52,7 +52,7 @@ class MinkowskiTest extends TestCase
         $this->assertEquals($expectedDistance, $actualDistance, '', $delta = 0.001);
     }
 
-    public function testCalculateDistanceForThreeDimensions()
+    public function testCalculateDistanceForThreeDimensions(): void
     {
         $a = [6, 10, 3];
         $b = [2, 5, 5];
@@ -63,7 +63,7 @@ class MinkowskiTest extends TestCase
         $this->assertEquals($expectedDistance, $actualDistance, '', $delta = 0.001);
     }
 
-    public function testCalculateDistanceForThreeDimensionsWithDifferentLambda()
+    public function testCalculateDistanceForThreeDimensionsWithDifferentLambda(): void
     {
         $distanceMetric = new Minkowski($lambda = 5);
 
diff --git a/tests/Phpml/Math/Kernel/RBFTest.php b/tests/Phpml/Math/Kernel/RBFTest.php
index 8076827..3ed7017 100644
--- a/tests/Phpml/Math/Kernel/RBFTest.php
+++ b/tests/Phpml/Math/Kernel/RBFTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class RBFTest extends TestCase
 {
-    public function testComputeRBFKernelFunction()
+    public function testComputeRBFKernelFunction(): void
     {
         $rbf = new RBF($gamma = 0.001);
 
diff --git a/tests/Phpml/Math/LinearAlgebra/EigenDecompositionTest.php b/tests/Phpml/Math/LinearAlgebra/EigenDecompositionTest.php
index 1db2c66..688874c 100644
--- a/tests/Phpml/Math/LinearAlgebra/EigenDecompositionTest.php
+++ b/tests/Phpml/Math/LinearAlgebra/EigenDecompositionTest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class EigenDecompositionTest extends TestCase
 {
-    public function testSymmetricMatrixEigenPairs()
+    public function testSymmetricMatrixEigenPairs(): void
     {
         // Acceptable error
         $epsilon = 0.001;
diff --git a/tests/Phpml/Math/MatrixTest.php b/tests/Phpml/Math/MatrixTest.php
index 0285b61..cd9fff2 100644
--- a/tests/Phpml/Math/MatrixTest.php
+++ b/tests/Phpml/Math/MatrixTest.php
@@ -12,12 +12,12 @@ class MatrixTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidMatrixSupplied()
+    public function testThrowExceptionOnInvalidMatrixSupplied(): void
     {
         new Matrix([[1, 2], [3]]);
     }
 
-    public function testCreateMatrixFromFlatArray()
+    public function testCreateMatrixFromFlatArray(): void
     {
         $flatArray = [1, 2, 3, 4];
         $matrix = Matrix::fromFlatArray($flatArray);
@@ -32,7 +32,7 @@ class MatrixTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\MatrixException
      */
-    public function testThrowExceptionOnInvalidColumnNumber()
+    public function testThrowExceptionOnInvalidColumnNumber(): void
     {
         $matrix = new Matrix([[1, 2, 3], [4, 5, 6]]);
         $matrix->getColumnValues(4);
@@ -41,13 +41,13 @@ class MatrixTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\MatrixException
      */
-    public function testThrowExceptionOnGetDeterminantIfArrayIsNotSquare()
+    public function testThrowExceptionOnGetDeterminantIfArrayIsNotSquare(): void
     {
         $matrix = new Matrix([[1, 2, 3], [4, 5, 6]]);
         $matrix->getDeterminant();
     }
 
-    public function testGetMatrixDeterminant()
+    public function testGetMatrixDeterminant(): void
     {
         //http://matrix.reshish.com/determinant.php
         $matrix = new Matrix([
@@ -68,7 +68,7 @@ class MatrixTest extends TestCase
         $this->assertEquals(1116.5035, $matrix->getDeterminant(), '', $delta = 0.0001);
     }
 
-    public function testMatrixTranspose()
+    public function testMatrixTranspose(): void
     {
         $matrix = new Matrix([
             [3, 3, 3],
@@ -88,7 +88,7 @@ class MatrixTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnMultiplyWhenInconsistentMatrixSupplied()
+    public function testThrowExceptionOnMultiplyWhenInconsistentMatrixSupplied(): void
     {
         $matrix1 = new Matrix([[1, 2, 3], [4, 5, 6]]);
         $matrix2 = new Matrix([[3, 2, 1], [6, 5, 4]]);
@@ -96,7 +96,7 @@ class MatrixTest extends TestCase
         $matrix1->multiply($matrix2);
     }
 
-    public function testMatrixMultiplyByMatrix()
+    public function testMatrixMultiplyByMatrix(): void
     {
         $matrix1 = new Matrix([
             [1, 2, 3],
@@ -117,7 +117,7 @@ class MatrixTest extends TestCase
         $this->assertEquals($product, $matrix1->multiply($matrix2)->toArray());
     }
 
-    public function testDivideByScalar()
+    public function testDivideByScalar(): void
     {
         $matrix = new Matrix([
             [4, 6, 8],
@@ -135,7 +135,7 @@ class MatrixTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\MatrixException
      */
-    public function testThrowExceptionWhenInverseIfArrayIsNotSquare()
+    public function testThrowExceptionWhenInverseIfArrayIsNotSquare(): void
     {
         $matrix = new Matrix([[1, 2, 3], [4, 5, 6]]);
         $matrix->inverse();
@@ -144,7 +144,7 @@ class MatrixTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\MatrixException
      */
-    public function testThrowExceptionWhenInverseIfMatrixIsSingular()
+    public function testThrowExceptionWhenInverseIfMatrixIsSingular(): void
     {
         $matrix = new Matrix([
           [0, 0, 0],
@@ -155,7 +155,7 @@ class MatrixTest extends TestCase
         $matrix->inverse();
     }
 
-    public function testInverseMatrix()
+    public function testInverseMatrix(): void
     {
         //http://ncalculators.com/matrix/inverse-matrix.htm
         $matrix = new Matrix([
@@ -173,7 +173,7 @@ class MatrixTest extends TestCase
         $this->assertEquals($inverseMatrix, $matrix->inverse()->toArray(), '', $delta = 0.0001);
     }
 
-    public function testCrossOutMatrix()
+    public function testCrossOutMatrix(): void
     {
         $matrix = new Matrix([
             [3, 4, 2],
@@ -189,14 +189,14 @@ class MatrixTest extends TestCase
         $this->assertEquals($crossOuted, $matrix->crossOut(1, 1)->toArray());
     }
 
-    public function testToScalar()
+    public function testToScalar(): void
     {
         $matrix = new Matrix([[1, 2, 3], [3, 2, 3]]);
 
         $this->assertEquals($matrix->toScalar(), 1);
     }
 
-    public function testMultiplyByScalar()
+    public function testMultiplyByScalar(): void
     {
         $matrix = new Matrix([
             [4, 6, 8],
@@ -211,7 +211,7 @@ class MatrixTest extends TestCase
         $this->assertEquals($result, $matrix->multiplyByScalar(-2)->toArray());
     }
 
-    public function testAdd()
+    public function testAdd(): void
     {
         $array1 = [1, 1, 1];
         $array2 = [2, 2, 2];
@@ -223,7 +223,7 @@ class MatrixTest extends TestCase
         $this->assertEquals($result, $m1->add($m2)->toArray()[0]);
     }
 
-    public function testSubtract()
+    public function testSubtract(): void
     {
         $array1 = [1, 1, 1];
         $array2 = [2, 2, 2];
@@ -235,7 +235,7 @@ class MatrixTest extends TestCase
         $this->assertEquals($result, $m1->subtract($m2)->toArray()[0]);
     }
 
-    public function testTransposeArray()
+    public function testTransposeArray(): void
     {
         $array = [
             [1, 1, 1],
@@ -250,7 +250,7 @@ class MatrixTest extends TestCase
         $this->assertEquals($transposed, Matrix::transposeArray($array));
     }
 
-    public function testDot()
+    public function testDot(): void
     {
         $vect1 = [2, 2, 2];
         $vect2 = [3, 3, 3];
diff --git a/tests/Phpml/Math/ProductTest.php b/tests/Phpml/Math/ProductTest.php
index 50ced9e..9c3a4b4 100644
--- a/tests/Phpml/Math/ProductTest.php
+++ b/tests/Phpml/Math/ProductTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class ProductTest extends TestCase
 {
-    public function testScalarProduct()
+    public function testScalarProduct(): void
     {
         $this->assertEquals(10, Product::scalar([2, 3], [-1, 4]));
         $this->assertEquals(-0.1, Product::scalar([1, 4, 1], [-2, 0.5, -0.1]));
diff --git a/tests/Phpml/Math/SetTest.php b/tests/Phpml/Math/SetTest.php
index 101763c..a572a42 100644
--- a/tests/Phpml/Math/SetTest.php
+++ b/tests/Phpml/Math/SetTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class SetTest extends TestCase
 {
-    public function testUnion()
+    public function testUnion(): void
     {
         $union = Set::union(new Set([3, 1]), new Set([3, 2, 2]));
 
@@ -18,7 +18,7 @@ class SetTest extends TestCase
         $this->assertEquals(3, $union->cardinality());
     }
 
-    public function testIntersection()
+    public function testIntersection(): void
     {
         $intersection = Set::intersection(new Set(['C', 'A']), new Set(['B', 'C']));
 
@@ -27,7 +27,7 @@ class SetTest extends TestCase
         $this->assertEquals(1, $intersection->cardinality());
     }
 
-    public function testDifference()
+    public function testDifference(): void
     {
         $difference = Set::difference(new Set(['C', 'A', 'B']), new Set(['A']));
 
@@ -36,7 +36,7 @@ class SetTest extends TestCase
         $this->assertEquals(2, $difference->cardinality());
     }
 
-    public function testPower()
+    public function testPower(): void
     {
         $power = Set::power(new Set(['A', 'B']));
 
@@ -45,7 +45,7 @@ class SetTest extends TestCase
         $this->assertCount(4, $power);
     }
 
-    public function testCartesian()
+    public function testCartesian(): void
     {
         $cartesian = Set::cartesian(new Set(['A']), new Set([1, 2]));
 
@@ -54,7 +54,7 @@ class SetTest extends TestCase
         $this->assertCount(2, $cartesian);
     }
 
-    public function testContains()
+    public function testContains(): void
     {
         $set = new Set(['B', 'A', 2, 1]);
 
@@ -65,21 +65,21 @@ class SetTest extends TestCase
         $this->assertFalse($set->containsAll(['A', 'B', 'C']));
     }
 
-    public function testRemove()
+    public function testRemove(): void
     {
         $set = new Set(['B', 'A', 2, 1]);
 
         $this->assertEquals((new Set([1, 2, 2, 2, 'B']))->toArray(), $set->remove('A')->toArray());
     }
 
-    public function testAdd()
+    public function testAdd(): void
     {
         $set = new Set(['B', 'A', 2, 1]);
         $set->addAll(['foo', 'bar']);
         $this->assertEquals(6, $set->cardinality());
     }
 
-    public function testEmpty()
+    public function testEmpty(): void
     {
         $set = new Set([1, 2]);
         $set->removeAll([2, 1]);
@@ -87,7 +87,7 @@ class SetTest extends TestCase
         $this->assertTrue($set->isEmpty());
     }
 
-    public function testToArray()
+    public function testToArray(): void
     {
         $set = new Set([1, 2, 2, 3, 'A', false, '', 1.1, -1, -10, 'B']);
 
diff --git a/tests/Phpml/Math/Statistic/CorrelationTest.php b/tests/Phpml/Math/Statistic/CorrelationTest.php
index dd01b8b..7fd2cec 100644
--- a/tests/Phpml/Math/Statistic/CorrelationTest.php
+++ b/tests/Phpml/Math/Statistic/CorrelationTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class CorrelationTest extends TestCase
 {
-    public function testPearsonCorrelation()
+    public function testPearsonCorrelation(): void
     {
         //http://www.stat.wmich.edu/s216/book/node126.html
         $delta = 0.001;
@@ -32,7 +32,7 @@ class CorrelationTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidArgumentsForPearsonCorrelation()
+    public function testThrowExceptionOnInvalidArgumentsForPearsonCorrelation(): void
     {
         Correlation::pearson([1, 2, 4], [3, 5]);
     }
diff --git a/tests/Phpml/Math/Statistic/CovarianceTest.php b/tests/Phpml/Math/Statistic/CovarianceTest.php
index d2abe7b..3a8c9d3 100644
--- a/tests/Phpml/Math/Statistic/CovarianceTest.php
+++ b/tests/Phpml/Math/Statistic/CovarianceTest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class CovarianceTest extends TestCase
 {
-    public function testSimpleCovariance()
+    public function testSimpleCovariance(): void
     {
         // Acceptable error
         $epsilon = 0.001;
diff --git a/tests/Phpml/Math/Statistic/GaussianTest.php b/tests/Phpml/Math/Statistic/GaussianTest.php
index a0c9700..8030f85 100644
--- a/tests/Phpml/Math/Statistic/GaussianTest.php
+++ b/tests/Phpml/Math/Statistic/GaussianTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class GaussianTest extends TestCase
 {
-    public function testPdf()
+    public function testPdf(): void
     {
         $std = 1.0;
         $mean = 0.0;
diff --git a/tests/Phpml/Math/Statistic/MeanTest.php b/tests/Phpml/Math/Statistic/MeanTest.php
index 8f6a8fe..86553e0 100644
--- a/tests/Phpml/Math/Statistic/MeanTest.php
+++ b/tests/Phpml/Math/Statistic/MeanTest.php
@@ -12,12 +12,12 @@ class MeanTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testArithmeticThrowExceptionOnEmptyArray()
+    public function testArithmeticThrowExceptionOnEmptyArray(): void
     {
         Mean::arithmetic([]);
     }
 
-    public function testArithmeticMean()
+    public function testArithmeticMean(): void
     {
         $delta = 0.01;
         $this->assertEquals(3.5, Mean::arithmetic([2, 5]), '', $delta);
@@ -28,19 +28,19 @@ class MeanTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testMedianThrowExceptionOnEmptyArray()
+    public function testMedianThrowExceptionOnEmptyArray(): void
     {
         Mean::median([]);
     }
 
-    public function testMedianOnOddLengthArray()
+    public function testMedianOnOddLengthArray(): void
     {
         $numbers = [5, 2, 6, 1, 3];
 
         $this->assertEquals(3, Mean::median($numbers));
     }
 
-    public function testMedianOnEvenLengthArray()
+    public function testMedianOnEvenLengthArray(): void
     {
         $numbers = [5, 2, 6, 1, 3, 4];
 
@@ -50,12 +50,12 @@ class MeanTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testModeThrowExceptionOnEmptyArray()
+    public function testModeThrowExceptionOnEmptyArray(): void
     {
         Mean::mode([]);
     }
 
-    public function testModeOnArray()
+    public function testModeOnArray(): void
     {
         $numbers = [5, 2, 6, 1, 3, 4, 6, 6, 5];
 
diff --git a/tests/Phpml/Math/Statistic/StandardDeviationTest.php b/tests/Phpml/Math/Statistic/StandardDeviationTest.php
index 35850c4..ead67b5 100644
--- a/tests/Phpml/Math/Statistic/StandardDeviationTest.php
+++ b/tests/Phpml/Math/Statistic/StandardDeviationTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class StandardDeviationTest extends TestCase
 {
-    public function testStandardDeviationOfPopulationSample()
+    public function testStandardDeviationOfPopulationSample(): void
     {
         //https://pl.wikipedia.org/wiki/Odchylenie_standardowe
         $delta = 0.001;
@@ -28,7 +28,7 @@ class StandardDeviationTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnEmptyArrayIfNotSample()
+    public function testThrowExceptionOnEmptyArrayIfNotSample(): void
     {
         StandardDeviation::population([], false);
     }
@@ -36,7 +36,7 @@ class StandardDeviationTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnToSmallArray()
+    public function testThrowExceptionOnToSmallArray(): void
     {
         StandardDeviation::population([1]);
     }
diff --git a/tests/Phpml/Metric/AccuracyTest.php b/tests/Phpml/Metric/AccuracyTest.php
index 02876e0..885893d 100644
--- a/tests/Phpml/Metric/AccuracyTest.php
+++ b/tests/Phpml/Metric/AccuracyTest.php
@@ -16,7 +16,7 @@ class AccuracyTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidArguments()
+    public function testThrowExceptionOnInvalidArguments(): void
     {
         $actualLabels = ['a', 'b', 'a', 'b'];
         $predictedLabels = ['a', 'a'];
@@ -24,7 +24,7 @@ class AccuracyTest extends TestCase
         Accuracy::score($actualLabels, $predictedLabels);
     }
 
-    public function testCalculateNormalizedScore()
+    public function testCalculateNormalizedScore(): void
     {
         $actualLabels = ['a', 'b', 'a', 'b'];
         $predictedLabels = ['a', 'a', 'b', 'b'];
@@ -32,7 +32,7 @@ class AccuracyTest extends TestCase
         $this->assertEquals(0.5, Accuracy::score($actualLabels, $predictedLabels));
     }
 
-    public function testCalculateNotNormalizedScore()
+    public function testCalculateNotNormalizedScore(): void
     {
         $actualLabels = ['a', 'b', 'a', 'b'];
         $predictedLabels = ['a', 'b', 'b', 'b'];
@@ -40,7 +40,7 @@ class AccuracyTest extends TestCase
         $this->assertEquals(3, Accuracy::score($actualLabels, $predictedLabels, false));
     }
 
-    public function testAccuracyOnDemoDataset()
+    public function testAccuracyOnDemoDataset(): void
     {
         $dataset = new RandomSplit(new IrisDataset(), 0.5, 123);
 
diff --git a/tests/Phpml/Metric/ClassificationReportTest.php b/tests/Phpml/Metric/ClassificationReportTest.php
index 7b142cf..b7ff02d 100644
--- a/tests/Phpml/Metric/ClassificationReportTest.php
+++ b/tests/Phpml/Metric/ClassificationReportTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class ClassificationReportTest extends TestCase
 {
-    public function testClassificationReportGenerateWithStringLabels()
+    public function testClassificationReportGenerateWithStringLabels(): void
     {
         $labels = ['cat', 'ant', 'bird', 'bird', 'bird'];
         $predicted = ['cat', 'cat', 'bird', 'bird', 'ant'];
@@ -29,7 +29,7 @@ class ClassificationReportTest extends TestCase
         $this->assertEquals($average, $report->getAverage(), '', 0.01);
     }
 
-    public function testClassificationReportGenerateWithNumericLabels()
+    public function testClassificationReportGenerateWithNumericLabels(): void
     {
         $labels = [0, 1, 2, 2, 2];
         $predicted = [0, 0, 2, 2, 1];
@@ -49,7 +49,7 @@ class ClassificationReportTest extends TestCase
         $this->assertEquals($average, $report->getAverage(), '', 0.01);
     }
 
-    public function testPreventDivideByZeroWhenTruePositiveAndFalsePositiveSumEqualsZero()
+    public function testPreventDivideByZeroWhenTruePositiveAndFalsePositiveSumEqualsZero(): void
     {
         $labels = [1, 2];
         $predicted = [2, 2];
@@ -59,7 +59,7 @@ class ClassificationReportTest extends TestCase
         $this->assertEquals([1 => 0.0, 2 => 0.5], $report->getPrecision(), '', 0.01);
     }
 
-    public function testPreventDivideByZeroWhenTruePositiveAndFalseNegativeSumEqualsZero()
+    public function testPreventDivideByZeroWhenTruePositiveAndFalseNegativeSumEqualsZero(): void
     {
         $labels = [2, 2, 1];
         $predicted = [2, 2, 3];
@@ -69,7 +69,7 @@ class ClassificationReportTest extends TestCase
         $this->assertEquals([1 => 0.0, 2 => 1, 3 => 0], $report->getPrecision(), '', 0.01);
     }
 
-    public function testPreventDividedByZeroWhenPredictedLabelsAllNotMatch()
+    public function testPreventDividedByZeroWhenPredictedLabelsAllNotMatch(): void
     {
         $labels = [1, 2, 3, 4, 5];
         $predicted = [2, 3, 4, 5, 6];
diff --git a/tests/Phpml/Metric/ConfusionMatrixTest.php b/tests/Phpml/Metric/ConfusionMatrixTest.php
index 1fa3130..f958bdf 100644
--- a/tests/Phpml/Metric/ConfusionMatrixTest.php
+++ b/tests/Phpml/Metric/ConfusionMatrixTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class ConfusionMatrixTest extends TestCase
 {
-    public function testComputeConfusionMatrixOnNumericLabels()
+    public function testComputeConfusionMatrixOnNumericLabels(): void
     {
         $actualLabels = [2, 0, 2, 2, 0, 1];
         $predictedLabels = [0, 0, 2, 2, 0, 2];
@@ -23,7 +23,7 @@ class ConfusionMatrixTest extends TestCase
         $this->assertEquals($confusionMatrix, ConfusionMatrix::compute($actualLabels, $predictedLabels));
     }
 
-    public function testComputeConfusionMatrixOnStringLabels()
+    public function testComputeConfusionMatrixOnStringLabels(): void
     {
         $actualLabels = ['cat', 'ant', 'cat', 'cat', 'ant', 'bird'];
         $predictedLabels = ['ant', 'ant', 'cat', 'cat', 'ant', 'cat'];
@@ -37,7 +37,7 @@ class ConfusionMatrixTest extends TestCase
         $this->assertEquals($confusionMatrix, ConfusionMatrix::compute($actualLabels, $predictedLabels));
     }
 
-    public function testComputeConfusionMatrixOnLabelsWithSubset()
+    public function testComputeConfusionMatrixOnLabelsWithSubset(): void
     {
         $actualLabels = ['cat', 'ant', 'cat', 'cat', 'ant', 'bird'];
         $predictedLabels = ['ant', 'ant', 'cat', 'cat', 'ant', 'cat'];
diff --git a/tests/Phpml/ModelManagerTest.php b/tests/Phpml/ModelManagerTest.php
index 400b6d1..e44a32a 100644
--- a/tests/Phpml/ModelManagerTest.php
+++ b/tests/Phpml/ModelManagerTest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class ModelManagerTest extends TestCase
 {
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         $filename = uniqid();
         $filepath = sys_get_temp_dir().DIRECTORY_SEPARATOR.$filename;
@@ -26,7 +26,7 @@ class ModelManagerTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\FileException
      */
-    public function testRestoreWrongFile()
+    public function testRestoreWrongFile(): void
     {
         $filepath = sys_get_temp_dir().DIRECTORY_SEPARATOR.'unexisting';
         $modelManager = new ModelManager();
diff --git a/tests/Phpml/NeuralNetwork/ActivationFunction/BinaryStepTest.php b/tests/Phpml/NeuralNetwork/ActivationFunction/BinaryStepTest.php
index a62b1c9..95e95cb 100644
--- a/tests/Phpml/NeuralNetwork/ActivationFunction/BinaryStepTest.php
+++ b/tests/Phpml/NeuralNetwork/ActivationFunction/BinaryStepTest.php
@@ -12,7 +12,7 @@ class BinaryStepTest extends TestCase
     /**
      * @dataProvider binaryStepProvider
      */
-    public function testBinaryStepActivationFunction($expected, $value)
+    public function testBinaryStepActivationFunction($expected, $value): void
     {
         $binaryStep = new BinaryStep();
 
diff --git a/tests/Phpml/NeuralNetwork/ActivationFunction/GaussianTest.php b/tests/Phpml/NeuralNetwork/ActivationFunction/GaussianTest.php
index 19b6cca..f7af7c0 100644
--- a/tests/Phpml/NeuralNetwork/ActivationFunction/GaussianTest.php
+++ b/tests/Phpml/NeuralNetwork/ActivationFunction/GaussianTest.php
@@ -12,7 +12,7 @@ class GaussianTest extends TestCase
     /**
      * @dataProvider gaussianProvider
      */
-    public function testGaussianActivationFunction($expected, $value)
+    public function testGaussianActivationFunction($expected, $value): void
     {
         $gaussian = new Gaussian();
 
diff --git a/tests/Phpml/NeuralNetwork/ActivationFunction/HyperboliTangentTest.php b/tests/Phpml/NeuralNetwork/ActivationFunction/HyperboliTangentTest.php
index b27dfa0..95f437f 100644
--- a/tests/Phpml/NeuralNetwork/ActivationFunction/HyperboliTangentTest.php
+++ b/tests/Phpml/NeuralNetwork/ActivationFunction/HyperboliTangentTest.php
@@ -12,7 +12,7 @@ class HyperboliTangentTest extends TestCase
     /**
      * @dataProvider tanhProvider
      */
-    public function testHyperbolicTangentActivationFunction($beta, $expected, $value)
+    public function testHyperbolicTangentActivationFunction($beta, $expected, $value): void
     {
         $tanh = new HyperbolicTangent($beta);
 
diff --git a/tests/Phpml/NeuralNetwork/ActivationFunction/PReLUTest.php b/tests/Phpml/NeuralNetwork/ActivationFunction/PReLUTest.php
index 5e14ce8..873520e 100644
--- a/tests/Phpml/NeuralNetwork/ActivationFunction/PReLUTest.php
+++ b/tests/Phpml/NeuralNetwork/ActivationFunction/PReLUTest.php
@@ -12,7 +12,7 @@ class PReLUTest extends TestCase
     /**
      * @dataProvider preluProvider
      */
-    public function testPReLUActivationFunction($beta, $expected, $value)
+    public function testPReLUActivationFunction($beta, $expected, $value): void
     {
         $prelu = new PReLU($beta);
 
diff --git a/tests/Phpml/NeuralNetwork/ActivationFunction/SigmoidTest.php b/tests/Phpml/NeuralNetwork/ActivationFunction/SigmoidTest.php
index 6894006..096a376 100644
--- a/tests/Phpml/NeuralNetwork/ActivationFunction/SigmoidTest.php
+++ b/tests/Phpml/NeuralNetwork/ActivationFunction/SigmoidTest.php
@@ -12,7 +12,7 @@ class SigmoidTest extends TestCase
     /**
      * @dataProvider sigmoidProvider
      */
-    public function testSigmoidActivationFunction($beta, $expected, $value)
+    public function testSigmoidActivationFunction($beta, $expected, $value): void
     {
         $sigmoid = new Sigmoid($beta);
 
diff --git a/tests/Phpml/NeuralNetwork/ActivationFunction/ThresholdedReLUTest.php b/tests/Phpml/NeuralNetwork/ActivationFunction/ThresholdedReLUTest.php
index 571a197..1800d7b 100644
--- a/tests/Phpml/NeuralNetwork/ActivationFunction/ThresholdedReLUTest.php
+++ b/tests/Phpml/NeuralNetwork/ActivationFunction/ThresholdedReLUTest.php
@@ -12,7 +12,7 @@ class ThresholdedReLUTest extends TestCase
     /**
      * @dataProvider thresholdProvider
      */
-    public function testThresholdedReLUActivationFunction($theta, $expected, $value)
+    public function testThresholdedReLUActivationFunction($theta, $expected, $value): void
     {
         $thresholdedReLU = new ThresholdedReLU($theta);
 
diff --git a/tests/Phpml/NeuralNetwork/LayerTest.php b/tests/Phpml/NeuralNetwork/LayerTest.php
index 4105b6f..284b1eb 100644
--- a/tests/Phpml/NeuralNetwork/LayerTest.php
+++ b/tests/Phpml/NeuralNetwork/LayerTest.php
@@ -11,14 +11,14 @@ use PHPUnit\Framework\TestCase;
 
 class LayerTest extends TestCase
 {
-    public function testLayerInitialization()
+    public function testLayerInitialization(): void
     {
         $layer = new Layer();
 
         $this->assertEquals([], $layer->getNodes());
     }
 
-    public function testLayerInitializationWithDefaultNodesType()
+    public function testLayerInitializationWithDefaultNodesType(): void
     {
         $layer = new Layer($number = 5);
 
@@ -28,7 +28,7 @@ class LayerTest extends TestCase
         }
     }
 
-    public function testLayerInitializationWithExplicitNodesType()
+    public function testLayerInitializationWithExplicitNodesType(): void
     {
         $layer = new Layer($number = 5, $class = Bias::class);
 
@@ -41,12 +41,12 @@ class LayerTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\InvalidArgumentException
      */
-    public function testThrowExceptionOnInvalidNodeClass()
+    public function testThrowExceptionOnInvalidNodeClass(): void
     {
         new Layer(1, \stdClass::class);
     }
 
-    public function testAddNodesToLayer()
+    public function testAddNodesToLayer(): void
     {
         $layer = new Layer();
         $layer->addNode($node1 = new Neuron());
diff --git a/tests/Phpml/NeuralNetwork/Network/LayeredNetworkTest.php b/tests/Phpml/NeuralNetwork/Network/LayeredNetworkTest.php
index 8b3f0e0..48bee66 100644
--- a/tests/Phpml/NeuralNetwork/Network/LayeredNetworkTest.php
+++ b/tests/Phpml/NeuralNetwork/Network/LayeredNetworkTest.php
@@ -11,7 +11,7 @@ use PHPUnit\Framework\TestCase;
 
 class LayeredNetworkTest extends TestCase
 {
-    public function testLayersSettersAndGetters()
+    public function testLayersSettersAndGetters(): void
     {
         $network = $this->getLayeredNetworkMock();
 
@@ -21,7 +21,7 @@ class LayeredNetworkTest extends TestCase
         $this->assertEquals([$layer1, $layer2], $network->getLayers());
     }
 
-    public function testGetLastLayerAsOutputLayer()
+    public function testGetLastLayerAsOutputLayer(): void
     {
         $network = $this->getLayeredNetworkMock();
         $network->addLayer($layer1 = new Layer());
@@ -32,7 +32,7 @@ class LayeredNetworkTest extends TestCase
         $this->assertEquals($layer2, $network->getOutputLayer());
     }
 
-    public function testSetInputAndGetOutput()
+    public function testSetInputAndGetOutput(): void
     {
         $network = $this->getLayeredNetworkMock();
         $network->addLayer(new Layer(2, Input::class));
diff --git a/tests/Phpml/NeuralNetwork/Node/BiasTest.php b/tests/Phpml/NeuralNetwork/Node/BiasTest.php
index 8f01d52..ee311f5 100644
--- a/tests/Phpml/NeuralNetwork/Node/BiasTest.php
+++ b/tests/Phpml/NeuralNetwork/Node/BiasTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class BiasTest extends TestCase
 {
-    public function testBiasOutput()
+    public function testBiasOutput(): void
     {
         $bias = new Bias();
 
diff --git a/tests/Phpml/NeuralNetwork/Node/InputTest.php b/tests/Phpml/NeuralNetwork/Node/InputTest.php
index 31d86c9..2d3be71 100644
--- a/tests/Phpml/NeuralNetwork/Node/InputTest.php
+++ b/tests/Phpml/NeuralNetwork/Node/InputTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class InputTest extends TestCase
 {
-    public function testInputInitialization()
+    public function testInputInitialization(): void
     {
         $input = new Input();
         $this->assertEquals(0.0, $input->getOutput());
@@ -18,7 +18,7 @@ class InputTest extends TestCase
         $this->assertEquals($value, $input->getOutput());
     }
 
-    public function testSetInput()
+    public function testSetInput(): void
     {
         $input = new Input();
         $input->setInput($value = 6.9);
diff --git a/tests/Phpml/NeuralNetwork/Node/Neuron/SynapseTest.php b/tests/Phpml/NeuralNetwork/Node/Neuron/SynapseTest.php
index 126d70d..02d6dfa 100644
--- a/tests/Phpml/NeuralNetwork/Node/Neuron/SynapseTest.php
+++ b/tests/Phpml/NeuralNetwork/Node/Neuron/SynapseTest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class SynapseTest extends TestCase
 {
-    public function testSynapseInitialization()
+    public function testSynapseInitialization(): void
     {
         $node = $this->getNodeMock($nodeOutput = 0.5);
 
@@ -25,7 +25,7 @@ class SynapseTest extends TestCase
         $this->assertInternalType('float', $synapse->getWeight());
     }
 
-    public function testSynapseWeightChange()
+    public function testSynapseWeightChange(): void
     {
         $node = $this->getNodeMock();
         $synapse = new Synapse($node, $weight = 0.75);
diff --git a/tests/Phpml/NeuralNetwork/Node/NeuronTest.php b/tests/Phpml/NeuralNetwork/Node/NeuronTest.php
index a7bc6fe..89f1ca1 100644
--- a/tests/Phpml/NeuralNetwork/Node/NeuronTest.php
+++ b/tests/Phpml/NeuralNetwork/Node/NeuronTest.php
@@ -11,7 +11,7 @@ use PHPUnit\Framework\TestCase;
 
 class NeuronTest extends TestCase
 {
-    public function testNeuronInitialization()
+    public function testNeuronInitialization(): void
     {
         $neuron = new Neuron();
 
@@ -19,7 +19,7 @@ class NeuronTest extends TestCase
         $this->assertEquals(0.5, $neuron->getOutput());
     }
 
-    public function testNeuronActivationFunction()
+    public function testNeuronActivationFunction(): void
     {
         $activationFunction = $this->getMockBuilder(BinaryStep::class)->getMock();
         $activationFunction->method('compute')->with(0)->willReturn($output = 0.69);
@@ -29,7 +29,7 @@ class NeuronTest extends TestCase
         $this->assertEquals($output, $neuron->getOutput());
     }
 
-    public function testNeuronWithSynapse()
+    public function testNeuronWithSynapse(): void
     {
         $neuron = new Neuron();
         $neuron->addSynapse($synapse = $this->getSynapseMock());
@@ -38,7 +38,7 @@ class NeuronTest extends TestCase
         $this->assertEquals(0.88, $neuron->getOutput(), '', 0.01);
     }
 
-    public function testNeuronRefresh()
+    public function testNeuronRefresh(): void
     {
         $neuron = new Neuron();
         $neuron->getOutput();
diff --git a/tests/Phpml/PipelineTest.php b/tests/Phpml/PipelineTest.php
index 7fc3545..fc06e56 100644
--- a/tests/Phpml/PipelineTest.php
+++ b/tests/Phpml/PipelineTest.php
@@ -17,7 +17,7 @@ use PHPUnit\Framework\TestCase;
 
 class PipelineTest extends TestCase
 {
-    public function testPipelineConstruction()
+    public function testPipelineConstruction(): void
     {
         $transformers = [
             new TfIdfTransformer(),
@@ -30,7 +30,7 @@ class PipelineTest extends TestCase
         $this->assertEquals($estimator, $pipeline->getEstimator());
     }
 
-    public function testPipelineEstimatorSetter()
+    public function testPipelineEstimatorSetter(): void
     {
         $pipeline = new Pipeline([new TfIdfTransformer()], new SVC());
 
@@ -40,7 +40,7 @@ class PipelineTest extends TestCase
         $this->assertEquals($estimator, $pipeline->getEstimator());
     }
 
-    public function testPipelineWorkflow()
+    public function testPipelineWorkflow(): void
     {
         $transformers = [
             new Imputer(null, new MostFrequentStrategy()),
@@ -68,7 +68,7 @@ class PipelineTest extends TestCase
         $this->assertEquals(4, $predicted[0]);
     }
 
-    public function testPipelineTransformers()
+    public function testPipelineTransformers(): void
     {
         $transformers = [
             new TokenCountVectorizer(new WordTokenizer()),
diff --git a/tests/Phpml/Preprocessing/ImputerTest.php b/tests/Phpml/Preprocessing/ImputerTest.php
index 2f5c31b..658b454 100644
--- a/tests/Phpml/Preprocessing/ImputerTest.php
+++ b/tests/Phpml/Preprocessing/ImputerTest.php
@@ -12,7 +12,7 @@ use PHPUnit\Framework\TestCase;
 
 class ImputerTest extends TestCase
 {
-    public function testComplementsMissingValuesWithMeanStrategyOnColumnAxis()
+    public function testComplementsMissingValuesWithMeanStrategyOnColumnAxis(): void
     {
         $data = [
             [1, null, 3, 4],
@@ -34,7 +34,7 @@ class ImputerTest extends TestCase
         $this->assertEquals($imputeData, $data, '', $delta = 0.01);
     }
 
-    public function testComplementsMissingValuesWithMeanStrategyOnRowAxis()
+    public function testComplementsMissingValuesWithMeanStrategyOnRowAxis(): void
     {
         $data = [
             [1, null, 3, 4],
@@ -56,7 +56,7 @@ class ImputerTest extends TestCase
         $this->assertEquals($imputeData, $data, '', $delta = 0.01);
     }
 
-    public function testComplementsMissingValuesWithMediaStrategyOnColumnAxis()
+    public function testComplementsMissingValuesWithMediaStrategyOnColumnAxis(): void
     {
         $data = [
             [1, null, 3, 4],
@@ -78,7 +78,7 @@ class ImputerTest extends TestCase
         $this->assertEquals($imputeData, $data, '', $delta = 0.01);
     }
 
-    public function testComplementsMissingValuesWithMediaStrategyOnRowAxis()
+    public function testComplementsMissingValuesWithMediaStrategyOnRowAxis(): void
     {
         $data = [
             [1, null, 3, 4],
@@ -100,7 +100,7 @@ class ImputerTest extends TestCase
         $this->assertEquals($imputeData, $data, '', $delta = 0.01);
     }
 
-    public function testComplementsMissingValuesWithMostFrequentStrategyOnColumnAxis()
+    public function testComplementsMissingValuesWithMostFrequentStrategyOnColumnAxis(): void
     {
         $data = [
             [1, null, 3, 4],
@@ -124,7 +124,7 @@ class ImputerTest extends TestCase
         $this->assertEquals($imputeData, $data);
     }
 
-    public function testComplementsMissingValuesWithMostFrequentStrategyOnRowAxis()
+    public function testComplementsMissingValuesWithMostFrequentStrategyOnRowAxis(): void
     {
         $data = [
             [1, null, 3, 4, 3],
@@ -148,7 +148,7 @@ class ImputerTest extends TestCase
         $this->assertEquals($imputeData, $data);
     }
 
-    public function testImputerWorksOnFitSamples()
+    public function testImputerWorksOnFitSamples(): void
     {
         $trainData = [
             [1, 3, 4],
diff --git a/tests/Phpml/Preprocessing/NormalizerTest.php b/tests/Phpml/Preprocessing/NormalizerTest.php
index 542eef5..22ed1bd 100644
--- a/tests/Phpml/Preprocessing/NormalizerTest.php
+++ b/tests/Phpml/Preprocessing/NormalizerTest.php
@@ -12,12 +12,12 @@ class NormalizerTest extends TestCase
     /**
      * @expectedException \Phpml\Exception\NormalizerException
      */
-    public function testThrowExceptionOnInvalidNorm()
+    public function testThrowExceptionOnInvalidNorm(): void
     {
         new Normalizer(99);
     }
 
-    public function testNormalizeSamplesWithL2Norm()
+    public function testNormalizeSamplesWithL2Norm(): void
     {
         $samples = [
             [1, -1, 2],
@@ -37,7 +37,7 @@ class NormalizerTest extends TestCase
         $this->assertEquals($normalized, $samples, '', $delta = 0.01);
     }
 
-    public function testNormalizeSamplesWithL1Norm()
+    public function testNormalizeSamplesWithL1Norm(): void
     {
         $samples = [
             [1, -1, 2],
@@ -57,7 +57,7 @@ class NormalizerTest extends TestCase
         $this->assertEquals($normalized, $samples, '', $delta = 0.01);
     }
 
-    public function testFitNotChangeNormalizerBehavior()
+    public function testFitNotChangeNormalizerBehavior(): void
     {
         $samples = [
             [1, -1, 2],
@@ -81,7 +81,7 @@ class NormalizerTest extends TestCase
         $this->assertEquals($normalized, $samples, '', $delta = 0.01);
     }
 
-    public function testL1NormWithZeroSumCondition()
+    public function testL1NormWithZeroSumCondition(): void
     {
         $samples = [
             [0, 0, 0],
@@ -101,7 +101,7 @@ class NormalizerTest extends TestCase
         $this->assertEquals($normalized, $samples, '', $delta = 0.01);
     }
 
-    public function testStandardNorm()
+    public function testStandardNorm(): void
     {
         // Generate 10 random vectors of length 3
         $samples = [];
diff --git a/tests/Phpml/Regression/LeastSquaresTest.php b/tests/Phpml/Regression/LeastSquaresTest.php
index ac457a9..7d835a2 100644
--- a/tests/Phpml/Regression/LeastSquaresTest.php
+++ b/tests/Phpml/Regression/LeastSquaresTest.php
@@ -10,7 +10,7 @@ use PHPUnit\Framework\TestCase;
 
 class LeastSquaresTest extends TestCase
 {
-    public function testPredictSingleFeatureSamples()
+    public function testPredictSingleFeatureSamples(): void
     {
         $delta = 0.01;
 
@@ -37,7 +37,7 @@ class LeastSquaresTest extends TestCase
         $this->assertEquals(278.66, $regression->predict([153260]), '', $delta);
     }
 
-    public function testPredictSingleFeatureSamplesWithMatrixTargets()
+    public function testPredictSingleFeatureSamplesWithMatrixTargets(): void
     {
         $delta = 0.01;
 
@@ -51,7 +51,7 @@ class LeastSquaresTest extends TestCase
         $this->assertEquals(4.06, $regression->predict([64]), '', $delta);
     }
 
-    public function testPredictMultiFeaturesSamples()
+    public function testPredictMultiFeaturesSamples(): void
     {
         $delta = 0.01;
 
@@ -68,7 +68,7 @@ class LeastSquaresTest extends TestCase
         $this->assertEquals(5711.40, $regression->predict([60000, 2000]), '', $delta);
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         //https://www.easycalculation.com/analytical/learn-least-square-regression.php
         $samples = [[60], [61], [62], [63], [65]];
diff --git a/tests/Phpml/Regression/SVRTest.php b/tests/Phpml/Regression/SVRTest.php
index 4ea19a3..3cd0ee5 100644
--- a/tests/Phpml/Regression/SVRTest.php
+++ b/tests/Phpml/Regression/SVRTest.php
@@ -11,7 +11,7 @@ use PHPUnit\Framework\TestCase;
 
 class SVRTest extends TestCase
 {
-    public function testPredictSingleFeatureSamples()
+    public function testPredictSingleFeatureSamples(): void
     {
         $delta = 0.01;
 
@@ -24,7 +24,7 @@ class SVRTest extends TestCase
         $this->assertEquals(4.03, $regression->predict([64]), '', $delta);
     }
 
-    public function testPredictMultiFeaturesSamples()
+    public function testPredictMultiFeaturesSamples(): void
     {
         $delta = 0.01;
 
@@ -37,7 +37,7 @@ class SVRTest extends TestCase
         $this->assertEquals([4109.82, 4112.28], $regression->predict([[60000, 1996], [60000, 2000]]), '', $delta);
     }
 
-    public function testSaveAndRestore()
+    public function testSaveAndRestore(): void
     {
         $samples = [[60], [61], [62], [63], [65]];
         $targets = [3.1, 3.6, 3.8, 4, 4.1];
diff --git a/tests/Phpml/SupportVectorMachine/DataTransformerTest.php b/tests/Phpml/SupportVectorMachine/DataTransformerTest.php
index 9aa1558..b1f8522 100644
--- a/tests/Phpml/SupportVectorMachine/DataTransformerTest.php
+++ b/tests/Phpml/SupportVectorMachine/DataTransformerTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class DataTransformerTest extends TestCase
 {
-    public function testTransformDatasetToTrainingSet()
+    public function testTransformDatasetToTrainingSet(): void
     {
         $samples = [[1, 1], [2, 1], [3, 2], [4, 5]];
         $labels = ['a', 'a', 'b', 'b'];
@@ -24,7 +24,7 @@ class DataTransformerTest extends TestCase
         $this->assertEquals($trainingSet, DataTransformer::trainingSet($samples, $labels));
     }
 
-    public function testTransformSamplesToTestSet()
+    public function testTransformSamplesToTestSet(): void
     {
         $samples = [[1, 1], [2, 1], [3, 2], [4, 5]];
 
diff --git a/tests/Phpml/SupportVectorMachine/SupportVectorMachineTest.php b/tests/Phpml/SupportVectorMachine/SupportVectorMachineTest.php
index 4cc6d57..ebbf99f 100644
--- a/tests/Phpml/SupportVectorMachine/SupportVectorMachineTest.php
+++ b/tests/Phpml/SupportVectorMachine/SupportVectorMachineTest.php
@@ -11,7 +11,7 @@ use PHPUnit\Framework\TestCase;
 
 class SupportVectorMachineTest extends TestCase
 {
-    public function testTrainCSVCModelWithLinearKernel()
+    public function testTrainCSVCModelWithLinearKernel(): void
     {
         $samples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
         $labels = ['a', 'a', 'a', 'b', 'b', 'b'];
@@ -35,7 +35,7 @@ SV
         $this->assertEquals($model, $svm->getModel());
     }
 
-    public function testPredictSampleWithLinearKernel()
+    public function testPredictSampleWithLinearKernel(): void
     {
         $samples = [[1, 3], [1, 4], [2, 4], [3, 1], [4, 1], [4, 2]];
         $labels = ['a', 'a', 'a', 'b', 'b', 'b'];
@@ -54,7 +54,7 @@ SV
         $this->assertEquals('b', $predictions[2]);
     }
 
-    public function testPredictSampleFromMultipleClassWithRbfKernel()
+    public function testPredictSampleFromMultipleClassWithRbfKernel(): void
     {
         $samples = [
             [1, 3], [1, 4], [1, 4],
@@ -85,7 +85,7 @@ SV
      * @expectedException \Phpml\Exception\InvalidArgumentException
      * @expectedExceptionMessage is not writable
      */
-    public function testThrowExceptionWhenVarPathIsNotWritable()
+    public function testThrowExceptionWhenVarPathIsNotWritable(): void
     {
         $svm = new SupportVectorMachine(Type::C_SVC, Kernel::RBF);
         $svm->setVarPath('var-path');
@@ -95,7 +95,7 @@ SV
      * @expectedException \Phpml\Exception\InvalidArgumentException
      * @expectedExceptionMessage does not exist
      */
-    public function testThrowExceptionWhenBinPathDoesNotExist()
+    public function testThrowExceptionWhenBinPathDoesNotExist(): void
     {
         $svm = new SupportVectorMachine(Type::C_SVC, Kernel::RBF);
         $svm->setBinPath('bin-path');
@@ -105,7 +105,7 @@ SV
      * @expectedException \Phpml\Exception\InvalidArgumentException
      * @expectedExceptionMessage not found
      */
-    public function testThrowExceptionWhenFileIsNotFoundInBinPath()
+    public function testThrowExceptionWhenFileIsNotFoundInBinPath(): void
     {
         $svm = new SupportVectorMachine(Type::C_SVC, Kernel::RBF);
         $svm->setBinPath('var');
diff --git a/tests/Phpml/Tokenization/WhitespaceTokenizerTest.php b/tests/Phpml/Tokenization/WhitespaceTokenizerTest.php
index 97fa833..f9d7c73 100644
--- a/tests/Phpml/Tokenization/WhitespaceTokenizerTest.php
+++ b/tests/Phpml/Tokenization/WhitespaceTokenizerTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class WhitespaceTokenizerTest extends TestCase
 {
-    public function testTokenizationOnAscii()
+    public function testTokenizationOnAscii(): void
     {
         $tokenizer = new WhitespaceTokenizer();
 
@@ -24,7 +24,7 @@ class WhitespaceTokenizerTest extends TestCase
         $this->assertEquals($tokens, $tokenizer->tokenize($text));
     }
 
-    public function testTokenizationOnUtf8()
+    public function testTokenizationOnUtf8(): void
     {
         $tokenizer = new WhitespaceTokenizer();
 
diff --git a/tests/Phpml/Tokenization/WordTokenizerTest.php b/tests/Phpml/Tokenization/WordTokenizerTest.php
index 09db222..607c327 100644
--- a/tests/Phpml/Tokenization/WordTokenizerTest.php
+++ b/tests/Phpml/Tokenization/WordTokenizerTest.php
@@ -9,7 +9,7 @@ use PHPUnit\Framework\TestCase;
 
 class WordTokenizerTest extends TestCase
 {
-    public function testTokenizationOnAscii()
+    public function testTokenizationOnAscii(): void
     {
         $tokenizer = new WordTokenizer();
 
@@ -24,7 +24,7 @@ class WordTokenizerTest extends TestCase
         $this->assertEquals($tokens, $tokenizer->tokenize($text));
     }
 
-    public function testTokenizationOnUtf8()
+    public function testTokenizationOnUtf8(): void
     {
         $tokenizer = new WordTokenizer();