feat: check variable usage with VariableAnalysis ruleset

Author: astratagem

Kleinweb/ruleset.xml

@@ -57,6 +57,17 @@ SPDX-License-Identifier: GPL-3.0-or-later AND MIT
 
     <!-- === Rules ============================================================================= -->
 
+    <!-- Detect undefined and unused variables. -->
+    <rule ref="VariableAnalysis"/>
+    <rule ref="VariableAnalysis.CodeAnalysis.VariableAnalysis">
+        <properties>
+            <!-- <https://github.com/sirbrillig/phpcs-variable-analysis#:~:text=allowWordPressPassByRefFunctions> -->
+            <property name="allowWordPressPassByRefFunctions" value="true"/>
+            <!-- Allow unused variables with the underscore prefix (e.g. `$_fnord`) -->
+            <property name="ignoreUnusedRegexp" value="/^_/"/>
+        </properties>
+    </rule>
+
     <!--
         We are not hosted on WordPress VIP, but their standards generally reflect
         best practices in WordPress development.  While the rules tend to reflect

composer.json

@@ -8,6 +8,7 @@
     "php": ">=8.1",
     "automattic/vipwpcs": "^3.0",
     "dealerdirect/phpcodesniffer-composer-installer": "*",
+    "sirbrillig/phpcs-variable-analysis": "^2.12",
     "slevomat/coding-standard": "^8.16.0",
     "squizlabs/php_codesniffer": "^3.7.2",
     "wp-coding-standards/wpcs": "^3.0"