Add GitHub Action for PHP syntax linting.

Author: Potherca

.github/workflows/linting.yml

@@ -13,3 +13,9 @@ jobs:
       - uses: "docker://pipelinecomponents/jsonlint:latest"
         with:
           args: "find . -not -path './.git/*' -name '*.json' -type f"
+
+  lint-php:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: pipeline-components/php-linter@master