It specifies which errors are occurred.

Example

<?php
error_reporting(E_ALL);
?>

E_ERROR ,E_WARNING,E_PARSE,E_NOTICE,E_CORE_ERROR,E_CORE_WARNING ,E_COMPILE_ERROR,E_COMPILE_WARNING,E_USER_ERROR,E_USER_WARNING,E_USER_NOTICE,E_STRICT ,E_RECOVERABLE_ERROR,E_ALL

Output

It shows all the errors occurred including waring,notice etc