PHP:服务器检查用户是否登录或退出

I'm wanting to be able to tell if a user is logged in or not from the server end. I currently have it set up where each user record in my database has an attribute 'loggedin' (boolean). If my user goes through the login process then he/she's 'loggedin' is set to '1'(true). If the user goes through the logout page I set it to '0'(false). My problem is what if they don't go through the logout page but instead their cookie times out?I'm wanting to be able to tell if a user is log