AJAX lädt php, welche eine Session setzt. Laden der Session funzt nicht!

<script>
(AJAX script blabla...)
function login(){
    var username = document.getElementById("username").value;  
    var password = document.getElementById("password").value;  
    var request = "ajaxTest2.php?u="+username+"&p="+password+"&a=login";  
var xmlhttp;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");  
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
      document.getElementById("display1").innerHTML=xmlhttp.responseText;  
    }
  }
xmlhttp.open("GET",request,true);  
xmlhttp.send();
}</script>

(LogIn script blabla)
if (login == true) { (jaja... nur als beispiel)
$_SESSION['loggedIn'] == true;  
}

(...)
function deleteFile($filename) {
if ($_SESSION['loggedIn']) {  
if ($filename != "" && file_exists('files/'.$filename)) {  
unlink('files/'.$filename);  
}
else {
$_SESSION['errorMsg'] = '<span class="error">Cannot delete file!</span>';  
}
}
else {
echo '<span class="error">Sie sind nicht eingeloggt</span>';  
}
}
(...)