The easiest way to develop asynchronous Ajax applications with PHP
 
xajax is an open source PHP class library that allows you to easily create powerful, web-based, Ajax  applications using HTML, CSS, JavaScript, and PHP.  Applications developed with xajax can asynchronously call  server-side PHP functions and update content without reloading the page.
 
How does xajax work?
 The xajax PHP object generates JavaScript wrapper functions for the PHP functions you want to be able to call asynchronously from your application.  When called, these wrapper functions use JavaScript's XMLHttpRequest object to  asynchronously communicate with the xajax object on the server which calls the corresponding PHP functions. Upon  completion, an xajax XML response is returned from the PHP functions, which xajax passes back to the application.  The XML response contains instructions and data that are parsed by xajax's JavaScript message pump and used to update  the content of your application.