- unserialize: documentation ( source)
<?php
$cookie = "O%3A11%3A%22SERIAL_User%22%3A3%3A%7Bs%3A8%3A%22username%22%3Bs%3A4%3A%22test%22%3Bs%3A8%3A%22password%22%3Bs%3A8%3A%22testtest%22%3Bs%3A9%3A%22userlevel%22%3Bi%3A0%3B%7D";
$pt = unserialize($cookie);
echo $pt
?>