您正在使用session，如:

if($_SESSION["user_level"] == 0)
{
    header('Location: index.php');
}

，但会话未在此处开始.在第一行开始会话，如:

but session is not started here. Start the session on first line like:

session_start();

只有在此之后，您才能访问会话变量.

only after that you can access the session variable.