после
mysql_query("UPDATE `users` SET `set_user` = '" . mysql_real_escape_string(serialize($set_user)) . "' WHERE `id` = '$user_id' LIMIT 1");вставилheader('Location: my_set.php');
exit; после
mysql_query("UPDATE `users` SET `set_user` = '" . mysql_real_escape_string(serialize($set_user)) . "' WHERE `id` = '$user_id' LIMIT 1");вставилheader('Location: my_set.php');
exit;