The open source AMFPHP project allows you, for free, to implement Flash Remoting with both PHP classes and SOAP-based web services. While not yet complete, AMFPHP is already very useful. This is an exciting new option for Flash developers who don't have access to a gateway server and for developers who program exclusively in PHP. The way that you utilize AMFPHP from Flash is no different than the other gateway implementations, with the exception of arguments when dealing with web services.
Please remember that AMFPHP is a rapidly evolving project and the information presented here is highly subject to change. For the most recent information about AMFPHP be sure to visit the project's site at http://www.amfphp.org.