Johannes Schluter has shared a handy mysqlnd plugin that he's written up that does two common things at once when you're pulling data from your database and pushing it into JSON - a mysqlnd_query_to_json function.
In my spare time I'm currently writing a shiny Web 2.0 application where I'm heavily using AJAX-like things, so what I do quite often in this application is, basically this: Check some pre-conditions (permissions etc.) then select some data from the database, do a fetch_all to get the complete result set as an array and run it through json_encode; Of course that example is simplified as I'm using the Symfony 2 framework for this project. [...] So I wrote a mysqlnd plugin.
His plugin (that he admits is "a hack") gives you the mysqlnd_query_to_json function that automagically transforms the results from the passed in query into JSON output. He notes some things lacking - like Unicode encoding and no MySQL bitfield support.