An uncaught exception server side should cause the response to be an 'error 500 internal server error'. I think PHP generates its own descriptive error pages; but it still may respond with a status other than 200.. If you didn't know; every response from a server starts with a 'header' which has important information; a status code, the page's content-type, user-provided data, and a few other things. We don't see that in view-source, and PHP doesn't force you to consider it, but it's there. Use this http://web-sniffer.net/ to view the header of a page that raises an error. See if the HTTP status code is something other than "HTTP/1.1 200 OK". The property ajaxobject.status returns the number of the status code: in this case 200. You can't split up the response from a server into 'errors' or 'non-errors' unless you define how to split it up. There is no such thing as an 'error' in a valid response : it is all basically plain text. The header tells you what the response should be treated as. [ and you can, via PHP, set the response to have any header/status you desire ]