头位置PHP问题

这是我的代码，它在localhost上工作并将我重定向到登录页面:

require("common.php"); 
unset($_SESSION['user']); 
 session_destroy(); 
header("Location: login.php"); 

但是当我把它上传到我的web服务器时，头没有发送。用户登录时没有重定向到我的登录页面。但是会话已经开始了。

代码如下:

require("common.php"); 

$submitted_username = ''; 

if(!empty($_POST)) 
{ 
    $query = " 
        SELECT 
            id, 
            username, 
            password, 
            salt, 
            email 
        FROM users 
        WHERE 
            username = :username 
    "; 

    $query_params = array( 
        ':username' => $_POST['username'] 
    ); 
    try 
    { 
        $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'])
        { 
            $login_ok = true; 
        } 
      if($login_ok) 
    { 
        unset($row['salt']); 
        unset($row['password']); 

        $_SESSION['user'] = $row; 
     header("Location:form_menu.php"); 
    } 
    else 
    { 
        echo("<font color=red>Login Failed. Icorrect Password </font>"); 

        $submitted_username = htmlentities($_POST['username'], ENT_QUOTES, 'UTF-8'); 
    }
    }
}

如何解决这个问题?我的网络服务器有问题吗?