Using the AMF protocol to communicate between a Flex/Flash application is the most efficient and recommendable way. AMF is an open binary protocol (specification can be found here) and allows you to directly work with primitive types between your server backend and your flex application in ActionScript.

Several projects exist to use this protocol with PHP (Zend_AMF, AMFPHP, SabreAMF) but Adobe encourages use of Zend_AMF.

This extension uses Zend_AMF (bundled in the extension) and provides an interface (or connector) to use the AMF protocol with eZ Publish.