From a brief read of your comments on the other site, it sounds like you're sending back just the value of $data in response to the AJAX subrequest. Doing so would be incorrect. AJAX responses are complete HTTP responses and need to have the appropriate HTTP headers (most significantly Content-Type, but you may also want to set others depending on the situation) and a trailing newline sent prior to their body, just like any other content transferred via HTTP.

Kinda vague, I know, but a) you haven't provided enough information yet to easily determine a more concrete answer and b) if you can't be bothered to show all relevant code, the text of the response you're currently trying to send (i.e., the value of $data plus anything else you're sending along with it), or even to repost your question here, then I'm not going to make the effort of looking that closely at your question or otherwise attempting to dig up the additional information required to provide a better answer.