Update php.yml

Add PHP 8.1

Author: jbelien

.github/workflows/php.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        php-version: ['7.3', '7.4', '8.0']
+        php-version: ['7.3', '7.4', '8.0', '8.1']
 
     steps:
     - uses: actions/checkout@v2