On his Adobe blog, Charlie Key has a new post demonstrating how you can transmit data back and forth between a Flex application and PHP using JSON messages.

In almost every RIA data needAdobe Flex makes implementing this solution an easy task. The solution explained in this tutorial uses PHP for the server-side programming and sends the data using JSON (JavaScript Object Notation) encoding. By the end of the tutorial you will be sending simple objects along with arrays of objects from your PHP code to your Flex client.s to be transmitted from a server to the client. [...]

There's a list of things you'll need to get started, the files you'll need to work with in the tutorial, and the steps to follow to make things work together nicely. Plenty of PHP and Flex code is included.