FLOCK() detects a lot of users , reduces the trapped errors significantly!

If you scan a series of files, ou are pretty sure to find out if anyone else is on your app!

You could also add a message table where active users lock their own records,
again not perfect, but both go a long way to eliminating the problems. The table also can be used as a tattle tale file to tell you if users are not logging out at the end of the day!

Mike