problem se session nedari se mi kontrola

<?php ob_start();

include './db.class.php';

session_start();

//Check whether the session variable
//SESS_MEMBER_ID is present or not

if(!isset($_SESSION['user_id']) || (trim($_SESSION['user_id'])=='')) 
{
header("location:login.php");
exit();
}

$db = new database('portfolio', 'localhost', 'root', ''); // new instance of database class
?>

  else
  {
  $user_id = sha1($user_id.$hash);
   /*** set the session user_id variable ***/
  session_regenerate_id();

	//Write session to disc
   
  $_SESSION['user_id'] = $user_id;
  
  	session_write_close();
//  $message = "$user_id";
  
/*** tell the user we are logged in ***/
  
     header("location:admin.php");
	exit();     

  $user_id = sha1($user_id.$hash);
   /*** set the session user_id variable ***/
  session_regenerate_id();

	//Write session to disc
   
  $_SESSION['user_id'] = $user_id;
session_write_close();
$message = $_SESSION['user_id'];
echo $message;


 
/*** tell the user we are logged in ***/
   
    // header("location:admin.php");
	exit();     
}