Add documentation for WordPress.NamingConventions.ValidVariableName

WordPress/Docs/NamingConventions/ValidVariableNameStandard.xml

@@ -0,0 +1,39 @@
+<?xml version="1.0"?>
+<documentation xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:noNamespaceSchemaLocation="https://phpcsstandards.github.io/PHPCSDevTools/phpcsdocs.xsd"
+    title="Valid Variable Name"
+>
+    <standard>
+    <![CDATA[
+    Use lowercase letters in variable names. Separate words using underscores.
+    ]]>
+    </standard>
+    <code_comparison>
+        <code title="Valid: lowercase variable name.">
+        <![CDATA[
+<em>$prefix_variable_name</em> = 'value';
+        ]]>
+        </code>
+        <code title="Invalid: mixed case function name.">
+        <![CDATA[
+<em>$Prefix_Variable_NAME</em> = 'value';
+        ]]>
+        </code>
+    </code_comparison>
+    <code_comparison>
+        <code title="Valid: words separated by underscores.">
+        <![CDATA[
+class My_Class {
+    public $variable_name = 'value';
+}
+        ]]>
+        </code>
+        <code title="Invalid: using camel case to separate words.">
+        <![CDATA[
+class My_Class {
+    public $variableName = 'value';
+}
+        ]]>
+        </code>
+    </code_comparison>
+</documentation>