- var_dump: documentation ( source)
- array_intersect: documentation ( source)
- array_keys: documentation ( source)
<?php
$_POST = array('email' => 'abc', 'phone' => '01234');
$possible = array('email', 'phone', 'foo', 'bar', 'baz');
$existing = array_intersect($possible, array_keys($_POST));
var_dump($existing);