- var_dump: documentation ( source)
- array_flip: documentation ( source)
- array_diff_key: documentation ( source)
<?php
$requiredAttributes = array('email', 'id', 'first_name', 'last_name');
$user= array('email' => 1, );
$missing = array_diff_key(array_flip($requiredAttributes), $user);
var_dump($missing);