ajax - meaningful errors

ajax - meaningful errors

I've been using jquery for a while and recently started using jquery ui and ajax, in conjunction with a php program I am developing. Everything is working well in all of the "normal" situations. However, I want to be able to detect situations that I can only do on the server side and communicate these as errors with an approriate message to the cliient side to be handled by the $.ajax error function. I know this is not a php forum, but someone must have done this before. If so, please give me some direction. I have been googling for hours and can't seem to find what I am looking for.

Just to be clear its the php I need help with, not the jquery.

Thanks!
Dean