PHP warnings happen... If you are seeing them on your webpage, I recommend that you shoot the developer an email so that the warnings can be addressed. Many times PHP versions on your server can cause warnings that the developer did not experience during development.
However, if your webpage is functioning properly, you may just want to get rid of that pesky PHP warning. You can insert the following snippet into the Prefix tab in the page inspector.
<?php error_reporting(E_ERROR | E_PARSE); ?>