Add php linting

Ensure PHP Code is valid for all supported PHP versions.

.github/workflows/ci.yaml

@@ -9,6 +9,25 @@ jobs:
       - name: Validate composer.json
         run: composer validate
 
+  php-linting:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        php-version:
+          - 7.3
+          - 7.4
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Install PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: "${{ matrix.php-version }}"
+
+      - name: PHP lint
+        run: "find *.php Classes Configuration Tests -name '*.php' -print0 | xargs -0 -n 1 -P 4 php -l"
+
   check-dependencies:
     runs-on: ubuntu-latest
     needs: [check-composer]