Travis: add PHP 8.1 support

See https://bugs.php.net/75474#1509646645 for more info on the static
change

.travis.yml

@@ -1,23 +1,30 @@
 language: php
 
-dist: xenial
-
-php:
-  - 5.6
-  - 7.0
-  - 7.1
-  - 7.2
-  - 7.3
-  - 7.4
-  - 8.0
-  - nightly
-
-before_install: true
-
 matrix:
+  include:
+    - php: 5.6
+      dist: xenial
+    - php: 7.0
+      dist: xenial
+    - php: 7.1
+      dist: xenial
+    - php: 7.2
+      dist: xenial
+    - php: 7.3
+      dist: xenial
+    - php: 7.4
+      dist: xenial
+    - php: 8.0
+      dist: bionic
+    - php: 8.1.0
+      dist: bionic
+    - php: nightly
+      dist: bionic
   allow_failures:
     - php: nightly
 
+before_install: true
+
 install:
   - phpenv config-rm xdebug.ini
   - eval `ssh-agent -s`

phpseclib/Crypt/EC/Formats/Keys/XML.php

@@ -152,7 +152,9 @@ abstract class XML
         }
         $node = $nodes->item(0);
         $ns_name = $node->lookupPrefix($ns);
-        $node->removeAttributeNS($ns, $ns_name);
+        if ($ns_name) {
+            $node->removeAttributeNS($ns, $ns_name);
+        }
         return $dom->saveXML($node);
     }
 

phpseclib/Math/BigInteger/Engines/PHP.php

@@ -578,6 +578,11 @@ abstract class PHP extends Engine
             $lhs =  new static();
             $rhs =  new static();
         }
+	if (static::class != get_class($temp)) {
+            $temp = new static();
+            $lhs =  new static();
+            $rhs =  new static();
+        }
         $temp_value = &$temp->value;
         $rhs_value =  &$rhs->value;