PHP session vars not saving

[audiodef]

I have an OO PHP program in which session vars are not being saved. The function in question is $user->login($email_id). Everything up to that point works. When I go into the login function and die($SESSION["user_id"]) I see what I expect to see. However, after the header() command, $SESSION["user_id"] is empty.

index.php:

require_once("init.php");

$option="";
$password_req="";
$email_req="";

extract($_GET,EXTR_IF_EXISTS);
extract($_POST,EXTR_IF_EXISTS);

include($user->docroot."/html/head.htm");

echo $_SESSION["user_id"]; //this now comes up emtpy!

switch($option){
  case 'login':
    if($_POST){
      $credentials=serialize(array("password"=>$password_req));
      if($login_id=$user->authenticate($credentials,"user")){
        if($user->emailIsValid($email_req)){
          $email_id=$user->collectEmail($link,$email_req,$login_id);
          $user->login($email_id); //SESSION DATA IS SAVED HERE - FUNCTION DEFINITION BELOW
        }else{
          include($user->docroot."/html/messages/invalid_email.htm");
          include($user->docroot."/html/user/login.htm");
        }
      }else{
        include($user->docroot."/html/messages/login_failed.htm");
        include($user->docroot."/html/user/login.htm");
      }
    }else{
      include($user->docroot."/html/user/login.htm");
    }
  break;
  default:
    include($user->docroot."/html/user/home.htm");
}

include($user->docroot."/html/foot.htm");

init.php:

header("Cache-Control: no-cache, must-revalidate");
header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");

session_save_path("/home/users/web/b2621/moo.studio5wjd/cgi-bin/tmp");
session_start();
ob_start();

set_time_limit(120);

require("dbconn.php");

require_once("classes/systemutils.class.php");
require_once("classes/user.class.php");

$user=new User;

function login()

  function login($id){
    /*
    Starts session with the provided id.
    */

$_SESSION["user_id"]=$id;
session_write_close();
header("location:".$_SERVER["PHP_SELF"]);
  }