Trent Richardson created a very small and simple JsonSQL library for JavaScript which allows you to run an extremely limited subset of SQL against a Json array/object. I quickly ported it over to PHP 5 and it works like a charm, although the syntax for the WHERE clause isn't exactly the same but the rest ported across properly.

He's included some sample source code of it in action - selecting information out of an array of user information, once changing their order and the other grabbing a username. Complete source for the library is included too.