diff --git a/phpseclib/Crypt/DH.php b/phpseclib/Crypt/DH.php
index 0cf0c428..9375ad33 100644
--- a/phpseclib/Crypt/DH.php
+++ b/phpseclib/Crypt/DH.php
@@ -243,7 +243,6 @@ abstract class DH extends AsymmetricKey
      * $length is in bits
      *
      * @param int $length optional
-     * @return DH\PrivateKey
      */
     public static function createKey(Parameters $params, int $length = 0): PrivateKey
     {
diff --git a/phpseclib/Crypt/DSA.php b/phpseclib/Crypt/DSA.php
index a5974718..2175e440 100644
--- a/phpseclib/Crypt/DSA.php
+++ b/phpseclib/Crypt/DSA.php
@@ -180,7 +180,6 @@ abstract class DSA extends AsymmetricKey
      * Returns the private key, from which the publickey can be extracted
      *
      * @param int[] ...$args
-     * @return DSA\PrivateKey
      */
     public static function createKey(...$args): PrivateKey
     {
diff --git a/phpseclib/Math/BigInteger.php b/phpseclib/Math/BigInteger.php
index 328818c6..58dcd511 100644
--- a/phpseclib/Math/BigInteger.php
+++ b/phpseclib/Math/BigInteger.php
@@ -31,8 +31,8 @@ namespace phpseclib3\Math;
 
 use phpseclib3\Exception\BadConfigurationException;
 use phpseclib3\Exception\InvalidArgumentException;
+use phpseclib3\Exception\UnexpectedValueException;
 use phpseclib3\Math\BigInteger\Engines\Engine;
-use UnexpectedValueException;
 
 /**
  * Pure-PHP arbitrary precision integer arithmetic library. Supports base-2, base-10, base-16, and base-256
diff --git a/phpseclib/bootstrap.php b/phpseclib/bootstrap.php
index 517106c3..7aededc9 100644
--- a/phpseclib/bootstrap.php
+++ b/phpseclib/bootstrap.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 /**
  * Bootstrapping File for phpseclib
  *
diff --git a/tests/Functional/Net/SFTPUserStoryTest.php b/tests/Functional/Net/SFTPUserStoryTest.php
index 689b58e7..ce2d5fdc 100644
--- a/tests/Functional/Net/SFTPUserStoryTest.php
+++ b/tests/Functional/Net/SFTPUserStoryTest.php
@@ -813,8 +813,10 @@ class SFTPUserStoryTest extends PhpseclibFunctionalTestCase
     public function testCallableGetWithLength($sftp)
     {
         $sftp->put('test.txt', 'zzzzz');
-        $sftp->get('test.txt', function ($data) {
+        $sftp->get('test.txt', function ($data): void {
         }, 0, 1);
         $this->assertTrue(true);
+
+        return $sftp;
     }
 }
diff --git a/tests/Unit/File/X509/X509Test.php b/tests/Unit/File/X509/X509Test.php
index 162422fc..d42f9619 100644
--- a/tests/Unit/File/X509/X509Test.php
+++ b/tests/Unit/File/X509/X509Test.php
@@ -1293,7 +1293,7 @@ BbNA6tFZAwLoX18R6yEmzHAQ+R2Eliiaz7mgQ+M2d0ec6qQJFoO7aJsX
         $this->assertIsArray($r);
     }
 
-    public function testWildcardCert()
+    public function testWildcardCert(): void
     {
         $cert = '-----BEGIN CERTIFICATE-----
 MIIKqDCCCZCgAwIBAgIQAZ3dCTUFVNcaZ4TM/m6DFTANBgkqhkiG9w0BAQsFADBY
@@ -1365,7 +1365,7 @@ dzXhMVAQYlpu27381/Ts2SuDx6v/cZ8lV8D5o/xTtCpWAnLxM2bxSyVnYbk=
     /**
      * @group github1943
      */
-    public function testWeirdCharsCert()
+    public function testWeirdCharsCert(): void
     {
         $cert = '-----BEGIN CERTIFICATE-----
 MIIDtTCCAp2gAwIBAgICECEwDQYJKoZIhvcNAQELBQAwYzELMAkGA1UEBhMCVVMx
diff --git a/tests/Unit/Math/PrimeFieldTest.php b/tests/Unit/Math/PrimeFieldTest.php
index 470ce815..19dbbe03 100644
--- a/tests/Unit/Math/PrimeFieldTest.php
+++ b/tests/Unit/Math/PrimeFieldTest.php
@@ -26,7 +26,7 @@ class PrimeFieldTest extends PhpseclibTestCase
     /**
      * @group github1929
      */
-    public function testGarbageCollectedToBytes()
+    public function testGarbageCollectedToBytes(): void
     {
         $blob = base64_decode('BFgsTFQeqKr0toyURbtT43INMDS7FTHjz3yn3MR1/Yv/pb2b9ZCYNQ/Tafe5hQpEJ4TpZOKfikP/hWZvFL8QCPgqbIGqw/KTfA==');
         $public = "\0" . substr($blob, 0, 49);