Try just using:
<?php
// Initialize the session.
session_start();
// Unset all of the session variables.
$_SESSION = array();
// If it's desired to kill the session, also delete the session cookie.
// Note: This will destroy the session, and not just the session data!
if (isset($_COOKIE[session_name()])) {
setcookie(session_name(), '', time()-42000, '/');
}
// Finally, destroy the session.
session_destroy();
?>
instead of what you have. (I got this directly from the session_destroy page at php.net)
to the top of the success page before the clear cart code.