[PHP] Problem

[Jorge]

I have a problem when I do login for go to the page of administration on my web, Go 
back and can't entry this is the code that control this. Where is the problem?

Thank you in advance

<?
        session_start();
        //echo session_id();

        if (!isset($_SESSION['id'])){

                echo("<script>top.location.href='../portada.php';</script>");
        }
?>