After spotting some null values in a few of his form fields following an Ajax request, Pablo Viquez decided to track down his issue:

While I was looking at some AJAX calls, I started to have a problem, for some reason, when I tried to query a JSON service I did using JQuery, the result was null for some fields. Going a little deeper, I notice that the records from the DB were OK, and the JavaScript was OK to, so what was the problem? The JSON Encode!

His issue stemmed from the character encoding of the string being passed into json_encode (in this case, UTF-8) was coming from a PHP script saved in a page encoded as ISO-8859-1. You can download the files he's come up with to illustrate the point.