Add support for PHP 8.1

Author: tomzx

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
         fail-fast: true
         matrix:
-            php: [7.3, 7.4, 8.0]
+            php: [7.3, 7.4, 8.0, 8.1]
             stability: [prefer-lowest, prefer-stable]
 
     name: PHP ${{ matrix.php }} - ${{ matrix.stability }}