If you're looking for the quickest way to get your PHP functionality Ajax enabled, you've got to take a look at PHPLiveX. It greatly simplifies the process of Ajaxifying your PHP code without knowing Javascript code.

He starts with a simple function to give you an idea of how things tie together. It creates a simple link ("Get Server Time") that fires off the Ajac request to the PHP backend, returning the local server's time - all with about 10 lines of PHP and HTML (well, the needed HTML).