In a new article to the PHP Zone (part of the DZone.com site) today Mihai Corlan has written up an introduction to the architecture of the interactions that happen between PHP and Flex applications.
When I did my first web project using Flex, boy oh boy, it was quite a switch! The clean separation between the client and the server, (the business logic on the client side in addition to business logic on the server side), client-side technology that is compiled instead of interpreted, two languages on the client, all these required a different mind-set from traditional web development. [...] I want to share with you some of the things that are specific to Flex in relation to PHP. And I want to show you how common tasks for building HTML/PHP applications are done with Flex and PHP.
He talks about the SOA architecture model that the Flex/PHP interaction follows (connecting to services) and illustrates a basic Flex client to PHP server request/response pattern for a simple application. He even includes a sample illustration of an AIR application's interaction with a backend PHP server.