Is it possible (and if so recommended) to use Flex remoting (mx.rpc) classes in an ActionScript only project?
I'm creating an application with login and registration to a PHP backend using AMFphp.
As an option I was wondering if it made sense to take advantage of the mx.rpc flex classes.
Actionscript only projects still use the flex framework, so you're fine. Are you trying to do it with the Flash IDE, or are you using FlexBuilder? If Flex Builder, then it doesn't matter at all.
You can use NetConnection class too.
You can use the entire RPC framework, including remote objects, but you have to manually initialize it by registering a number of class aliases (the Flex framework would do this for you, otherwise).
This great blog post explains the how and why.
If you're not really interested in the details and just want to get going, I wrote a tutorial on how to integrate BlazeDS with RobotLegs, which includes a framework-agnostic example you can use as a turnkey.
