Added PHPCompatibility check

Author: sreichel

.github/phpcs-ruleset.xml

@@ -20,4 +20,11 @@
         <exclude name="PSR12.ControlStructures.ControlStructureSpacing.LineIndent"/>
         <exclude name="PSR12.Files.FileHeader.SpacingAfterBlock"/>
     </rule>
+    <rule ref="PHPCompatibility" />
+    <rule ref="PHPCompatibility.FunctionNameRestrictions.ReservedFunctionNames.MethodDoubleUnderscore">
+        <exclude-pattern>*/Varien/Object.php*</exclude-pattern>
+        <exclude-pattern>*/Varien/Data/Form/Abstract.php*</exclude-pattern>
+        <exclude-pattern>*/Varien/Db/Tree.php*</exclude-pattern>
+        <exclude-pattern>*/Varien/Directory/Collection.php*</exclude-pattern>
+    </rule>
 </ruleset>