Prevent Multiple Login PHP

Does anybody have her own experience in implementing the mechanism to prevent multiple login in PHP and/or Javascript? I am looking for complete solution to prevent or reduce multiple user login to a web system. I mean using the same userID/password by different people. What I have know for now : - Very good solutions to monitor users' activity by writing session info in database (file). What I am looking for: - I've figured out that the most complicated part is to detect 'unclean logout' that is the situations when user did not log out with by 'Logout' button, and just 1. close the browser (I am looking for some Javascript code or any other idea) or 2. move to another web site from mine. What are the possible ways to detect unclean logouts? Is there any workaround available, like session timeout? How and when should I check the sessions for timeout if any? When should I allow the second (and more) login when the userID is mark as currently login? Sincerely, Eugene Voznesensky