In this article on the Symfony-Zone blog they show you how to use the sfAmfPlugin to let your framework application act as a backend for a Flex/Air application.

Today I want to show you how to use the sfAmfPlugin to create a simple Hello World application with Symfony, Flex and the plugin. AMF is the most comfortable way to communicate between a backend technology and Flex. AMF allows RMI (Remote Method Invocation) from Flex to a backend server that is able to support AMF. The sfAmfPlugin for Symfony adds AMF-support to the PHP-Framework.

Their example starts from the beginning - building a project, creating an AMF service for the front end to connect to and creating the controller/action that will return the "Hello World" message. They also include some of the Flex code to handle the request and response handling for the connection to the symfony backend.