On the Adobe Developer Connection site Richard Bates has recently posted a tutorial covering the integration of PHP and Flex as developed in Zend Studio/Flex Builder.
Tight integration between Flex and a server-side language means that your users will have an intuitive, engaging user experience with all the functionality they need. For developers, tight integration between PHP and the Flex development environments can provide big benefits by ensuring a smoother workflow for coding, debugging, and managing a project. You can make the most of such a unified environment by using Adobe Flex Builder and Zend Studio for Eclipse together.
He starts with the basics - installing everything and getting it all up and running, then making a sample "Hello World" application on both the PHP (backend) and the Flex (frontend) sides. Included in the tutorial is a look at sending XML out to the Flex application from PHP.