php session问题

session_start();if(Login() == true){  $_SESSION['login'] = 'ok';  //跳转到page.php}else{  echo "Login error";}

session_start();echo $_SESSION['login'];if(empty($_SESSION['login'])||$_SESSION['login']!='ok') //未登陆则跳回index{  ?><script>setTimeout('window.location="index.php"', 1000);</script><?}..........<script language="javascript">function goBack(){    AjaxTest();    window.location.href = "./index.php";}function AjaxTest()    {        xmlHttp=GetXmlHttpObject();        if (xmlHttp==null)        {            alert ("Browser does not support HTTP Request");            return;        }        var url = "ajax.php";        url += "?login=out";        xmlHttp.onreadystatechange=stateChanged         xmlHttp.open("GET",url,true)        xmlHttp.send(null)    }    function stateChanged()     {         if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")        {            alert(xmlHttp.responseText );        }     }    function GetXmlHttpObject()    {        var xmlHttp=null;        try        {            // Firefox, Opera 8.0+, Safari            xmlHttp=new XMLHttpRequest();        }        catch (e)        {            // Internet Explorer            try            {                xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");            }            catch (e)            {                xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");            }        }        return xmlHttp;    }</script>

<?    $login = $_GET["login"];        session_start();    unset($_SESSION['login']);     session_destroy();    echo "success";?>