Test multiple php versions

Author: thelfensdrfer

.github/workflows/phpunit.yml

@@ -10,6 +10,11 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php-versions: ['7.3', '7.3', '7.4', '8.0']
+
+    name: PHPUnit ${{ matrix.php-versions }}
 
     steps:
     - uses: actions/checkout@v2