You could alternatively use your own error handler in conjunction with trigger_error() instead of die() if all you're doing is looking for another way to show that something went wrong.

Never trust javascript to do anything that you need to make sure is done. Especially form validation - take care of everything server side, then if you want to add additional "usability", work your javascript or ajax into it afterwards.

Frameworks. Pssht.

It sounds like you're asking how to validate a form field, more than how to return the error from it though, is this correct?
-m