Locking and unlocking records

I need to implement database record locking and unlocking. I want to prevent multiple users of a content management system from editing the same record simultaneously, which can result in one user's edits being overwritten. I can use PHP to lock a record when it's opened, and unlock it when it's updated or if the user goes anywhere else within the content management system (or logs out), but I need to account for a user leaving the CMS directly from the edit screen. I'm not aware of any way to do that other than using a Javascript onunload event to open a new window, run a PHP script, and close the new window onload -- very nasty and unreliable with pop-up blocking so popular. Can anyone suggest an all-PHP way? TIA -- Lowell Allen