Restrict account access to single user

We're creating a password protected mysql database accesible and searchable via a PHP front end. Because this is a revenue-based site, and users buy a password for access, we're wondering what the best php/mysql mechanism would be to allow only one person to access their account at a time. In other words, how do we prevent two users from using the same password to access the same account at the same time? Preferably, the solution should be javascript- and cookie-free. We've thought about registering the login into the user's mysql account record, but how to reset the "logged in" field to show the user as logged out if their machine crashes, or if they forget to log out? Any pointers to relevant web sites appreciated. Thanks.