File: vendor/squizlabs/php_codesniffer/src/Standards/PEAR/Docs/NamingConventions/ValidVariableNameStandard.xml

<documentation title="Variable Names">
    <standard>
    <![CDATA[
    Private member variable names should be prefixed with an underscore and public/protected variable names should not.
    ]]>
    </standard>
    <code_comparison>
        <code title="Valid: Proper member variable names.">
        <![CDATA[
class Foo
{
    public $<em>publicVar</em>;
    protected $<em>protectedVar</em>;
    private $<em>_privateVar</em>;
}
        ]]>
        </code>
        <code title="Invalid: underscores used on public/protected variables and not used on private variables.">
        <![CDATA[
class Foo
{
    public $<em>_publicVar</em>;
    protected $<em>_protectedVar</em>;
    private $<em>privateVar</em>;
}
        ]]>
        </code>
    </code_comparison>
</documentation>