FLEX

The most important reference for flex is Adobe Flex 3.5 Language Reference online document: http://livedocs.adobe.com/flex/3/langref/index.html

AMFPHP

AMFPHP is a free open-source PHP implementation of the Action Message Format(AMF). With AMFPHP, Flex application can call methods in PHP on the server conveniently just as calling a local method. More details can be found here: http://www.amfphp.org/

Gal API

You should be familiar with OAuth: http://oauth.net/