AJAX Framework recommnedation for PHP/JSON

[lampip]

I am new to AJAX and would like to use it in my site. I am looking for a very simple, light weight framework to faciliate ajax in php, ideally using json. Primary goal is easy to use, low overhead, and minimum jscript libs to download to client.

[MisYu]

google for:
- SAJAX
- XAJAX
- XOAD
- Symfony
- CakePHP