会话数据不能从一页传送到另一页

当用户在我的网站上使用登录系统并且提交的凭据正确时，设置会话和会话数据。但是，会话数据不会从一个页面传递到另一个页面。在登录脚本中，我打印出会话数组，它显示一切都已设置好。但是，当我使用打印会话的相同代码转到另一个页面时，它显示会话中没有任何内容。(是的，我在每个需要它的php页面的开头插入session_start())

login。

<?php
require("config.php"); 
if(!empty($_POST)) 
{ 
    // This query retreives the user's information from the database using 
    // their username. 
    $query = " 
        SELECT 
            id, 
            username, 
            password, 
            salt, 
            email 
        FROM users 
        WHERE 
            username = :username 
    "; 
    // The parameter values 
    $query_params = array( 
        ':username' => $_POST['username'] 
    ); 
    try 
    { 
        // Execute the query against the database 
        $stmt = $db->prepare($query); 
        $result = $stmt->execute($query_params); 
    } 
    catch(PDOException $ex) 
    { 
        die("Failed to run query: " . $ex->getMessage()); 
    } 

    $login_ok = false; 

    $row = $stmt->fetch(); 
    if($row) 
    { 
        $check_password = hash('sha256', $_POST['password'] . $row['salt']); 
        for($round = 0; $round < 65536; $round++) 
        { 
            $check_password = hash('sha256', $check_password . $row['salt']); 
        } 
        if($check_password === $row['password']) 
        { 
            // If they do, then we flip this to true 
            $login_ok = true; 
        } 
    } 
    // If the user logged in successfully, then we send them to the private members-only page 
    // Otherwise, we display a login failed message and show the login form again 
    if($login_ok) 
    { 
        unset($row['salt']); 
        unset($row['password']);  
        $_SESSION['USER'] = $row; 

        /* print_r($_SESSION); */
        session_write_close();
        header("Location: index.php"); 
        // Redirect the user to the index page. 
        die("Redirecting to the home page."); 
    } 
    else 
    { 
        print("Login Failed."); 
        $submitted_username = htmlentities($_POST['username'], ENT_QUOTES, 'UTF-8'); 
        header("Location: login.php");
        exit;
    } 
} 
echo $message; 
?>

下一个脚本检查会话是否已设置。如果不是，它会将用户重定向到登录页面。

<?php
/*** begin the session ***/
session_start();
if(!isset($_SESSION['USER']))
{ 
    // If they are not, we redirect them to the login page. 
    header("Location: ../login.php"); 
    exit;
} 
?>