Tug already mentioned Basement, which will provide that functionality of "models" in the near future like you know it from ORM systems.
Aside from that, mapping your plain old php objects to JSON is very easy, thanks to the nature of json_encode/decode. Since you can pass it an arbitary object and it will store it as JSON, thats basically the only thing you need at hand. If you need more infos about JSON and PHP, my blog post is a good start: http://nitschinger.at/Handling-JSON-like-a-boss-in-PHP
If you use Basement, it makes it a little bit easier for you since it allows you to transform PHP types into JSON behind the scenes automatically (or write your own mapper if needed).
If you have a specific example that you want to build, let me know and I'd be happy to provide an example!