I have read on the adobe website that you could use PHP with flex.I know for applications to run on the desktop they have to be a .air format, like most tweeter desktop clients.I am therefore curious, won't one need things like Apache and MySQL for your app in PHP and flex or is everything transformed into a single .air including all your webserver.
PHP is used for the backend only. The client is written in Flex. Both communicate via AMF. So yes, you need a webserver and network connection for this. See The architecture of Flex and PHP applications