Create action to check against PSR12 coding standard

.github/workflows/phpcs_psr12.yml

@@ -0,0 +1,27 @@
+name: "CI"
+
+on:
+  pull_request:
+    paths:
+      - "html"
+
+jobs:
+  phpcs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0 # important!
+
+      # we may use whatever way to install phpcs, just specify the path on the next step
+      # however, curl seems to be the fastest
+      - name: Install PHP_CodeSniffer
+        run: |
+          curl -OL https://squizlabs.github.io/PHP_CodeSniffer/phpcs.phar
+          php phpcs.phar --version
+
+      - uses: tinovyatkin/action-php-codesniffer@v1
+        with:
+          files: "**.php" # you may customize glob as needed
+          phpcs_path: php phpcs.phar
+          standard: PSR12