问PHP $_SESSION不工作

EN

<?php
include('header.php');
if (isset($_SESSION['logged_in']))
{
    $link = 'profile.php';
    $link_name = 'Profile';
}
else
{
    $link = 'login.php';
    $link_name = 'Login';
}

if (isset($_POST['action']))
{
    $username = mysql_real_escape_string($_POST['username']);
    $password = mysql_real_escape_string($_POST['password']);
    $password = md5($password);
    $user = mysql_query("SELECT * FROM users WHERE `username`='".$username."'");
    if (mysql_num_rows($user) == 1)
    {
            while ($userinfo = mysql_fetch_array($user))
            {
                    if ($userinfo['banned'] != true)
                    {
                            $_SESSION['user_id'] = $userinfo['id'];
                            $_SESSION['username'] = $userinfo['username'];
                            $_SESSION['logged_in'] = "true";
                            header('Location: index.php');
                    }
                    else
                    {
                            header('Location: login.php?error=banned');
                    }
            }
    }
    else if (mysql_num_rows($user) == 0)
    {
            header('Location: login.php?error=not-found');
    }
}
?>