Something really interesting has just been released on CodePlex: PHP for Microsoft AJAX Library. The code you have to use in your scripts is clean and easy, providing a full-featured PHP backend to the Microsoft AJAX Library.
The Microsoft AJAX Library is a pure-JavaScript library that's used by ASP.NET AJAX but is also available as a separate download. Because it's pure JavaScript, it's not tied to ASP.NET on the backend. PHP for MS AJAX is code to help you make use of the Microsoft AJAX Library from PHP applications. This PHP script simply exposes PHP classes as AJAX-enabled web services, just like ASP.NET applications would do.