On the TechLabs website there's a recent post looking at how to get Flex Builder 3.0 and PHP to work together via the Zend_AMF component of the Zend Framework.

In this article I will explain how to build a simple adobe flex application that communicates with php using remote objects calls to the new AMF php implementation supported by adobe and called ZendAMF. In my example I will use Adobe Flex Builder 3, ZendAMF and a Apache web server with php support and mysql server on windows. If you do not have any available server with php (at least) you will need to install one on your machine. It’s easy, nowadays you have many standalone installers that do everything, just look at the wampserver for example.

If you want to get started right away, you can download the source and get going. Otherwise, you can check out the rest of the tutorial to see how they build up their example. They show how to create the project, set up the Zend_Amf extension's configuration and creating the simple application that echos back a "hello world" sort of response based on a text field's entry.