PHPStan: Add docs and infrastructure for local configuration (#4494)

.gitignore

@@ -52,6 +52,8 @@ captainhook.config.json
 captainhook.local.json
 .phpunit.result.cache
 .php-cs-fixer.cache
+phpstan.local.neon
+phpstan-baseline.neon
 
 # File Delivery
 override.php

docs/development/static_code_analysis.md

@@ -36,6 +36,45 @@ You can overwrite the rule level by passing a `--level` argument:
 ./CI/PHPStan/run_check.sh Services/Mail --level 3
 ```
 
+### Custom Configuration File
+
+A custom configuration file could be provided by adding a file named `phpstan.local.neon` to the ILIAS root directory.
+The filename is ignored by the `.gitignore` file and ILIAS `\RootFolderTest`, which ensures no new files are added
+to the root level of ILIAS.
+
+Example:
+
+```yaml
+includes:
+    - CI/PHPStan/phpstan.neon
+parameters:
+    level: 9
+```
+
+In this example the shared `PHPStan` configuration is extended by the local configuration, where we defined `stricter`
+rule level.
+
+### Baseline
+
+A [baseline file](https://phpstan.org/user-guide/baseline) can be generated by using the `--generate-baseline` option.
+
+```bash
+/CI/PHPStan/run_check.sh -c phpstan.local.neon Services/Mail --generate-baseline
+```
+
+The generated baseline file `phpstan-baseline.neon` will be stored in the ILIAS root folder and must be included in the
+local `phpstan.local.neon` configuration.
+
+```yaml
+includes:
+    - CI/PHPStan/phpstan.neon
+    - phpstan-baseline.neon
+parameters:
+    level: 9
+```
+
+If you analyse different components/folders with `PHPStan`, you will need to generate a new baseline file.
+
 ### IDE Integration
 
 #### PHPStorm

tests/App/RootFolderTest.php

@@ -14,6 +14,8 @@ final class RootFolderTest extends TestCase
         '.htaccess',
         '.phpunit.result.cache',
         'captainhook.local.json',
+        'phpstan.local.neon',
+        'phpstan-baseline.neon',
         '.php_cs.cache',
         'calendar.php',
         'captainhook.json',