Return error from PHP

[jcanker]

I'm new to AJAX and working on a project which returns customer contact info via XML.  I've got that part working properly, but now I'm working on the logged in/security part.

 

If $_SESSION['logged'] isn't set properly, I want the php script to die and return an error to the jQuery function that called it.  Do I have to do this in xml since the jQuery processing the request is expecting xml back, or is there a way I can utilize the HTTP request headers to have php send back a specific fail error?