When using a multi-site setup, You may want errors only to appear for one site. In that case, use $_SERVER[‘HTTP_HOST’] in an if statement to only display PHP error messages for the domain of your choice:
<?php if ($_SERVER['HTTP_HOST']==='specific.domain-you-want.here') { error_reporting(E_ALL); ini_set('display_errors', TRUE); ini_set('display_startup_errors', TRUE); } ?>