How should an AJAX endpoint signal that servicing the request has failed?

Question

What data do I need to send from my server (PHP) so that jQuery recognizes it as error and executes the error callback?

Answer 1

<?php
header('HTTP/1.1 500 Internal Server Error');
?>

Answer 2

You could send any of the 5xx Server Error codes listed here back in your HTTP header. http://en.wikipedia.org/wiki/List_of_HTTP_status_codes

Or else, you could just send back a message that states an error has occurred in the response, then check for that string on the client and handle that...