Add PHPStan checks

.github/workflows/qa.yml

@@ -0,0 +1,12 @@
+on: [push, pull_request]
+name: Quality assurance
+jobs:
+  phpstan:
+    name: PHPStan
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - name: PHPStan
+        uses: "docker://oskarstark/phpstan-ga"
+        with:
+          args: analyse

phpstan.neon.dist

@@ -0,0 +1,14 @@
+parameters:
+    level: 1
+
+    paths:
+        - src
+# Uncomment this, when phpstan extensions are allowed
+#        - tests
+
+    excludes_analyse:
+        - src/Test/AbstractWidgetTestCase.php
+        - tests/bootstrap.php
+
+    autoload_files:
+        - vendor/autoload.php