Enter PHP code to perform additional validation for this form. Include the &lt;?php ?&gt; tags. $form_id and $form_values are available variables. If validation fails, use the form_set_error function to prevent the form from being submitted. Use the same syntax as a _validate function used in the <a href="http://api.drupal.org/api/file/developer/topics/forms_api.html">Forms API</a>.