Brutally token from Sephiroth.it:
DevNet article with Serializer Class

Vera Fleischer wrote a very interesting article on devnet:
"Getting a Handle on Web Services in Macromedia Flash MX Professional 2004".
The article is about consuming web services using Flash, thanks to the built-in classes for webservices in MX 2004 pro version, and PHP ( with nusoap and serializerClass).
More in detail, the article describe how to consume a web service as a Flash client on one side, and how to build a simple web service using PHP (here).

There is also an interesting link about the comparison between XML, Web services and Remoting.