chore: add PHP 8.1 (#362)

Author: Sergiy Petrov

.github/workflows/tests.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php: [ "5.6", "7.0", "7.1", "7.2", "7.3", "7.4", "8.0" ]
+        php: [ "5.6", "7.0", "7.1", "7.2", "7.3", "7.4", "8.0", "8.1"]
     name: PHP ${{matrix.php }} Unit Test
     steps:
       - uses: actions/checkout@v2