In Ajax programming development, passing data back and forth between client and server is a common task. There are two major data formats that one can use for this purpose, namely XML and JSON. My personal preference is JSON because I find it's less verbose and easier to parse.
Although latest PHP provides a very handy function (json_encode) for outputting data in JSON format, for almost every Ajax request I handle on the server side, I have to routinely format the data in a certain way so I can use it conveniently on the client side. For example, to indicate whether it is a valid Ajax request, I embed an field called has_error in every response. I find it's getting annoying after a while, so I wrote a class to automate things a bit.