Kevin Schroeder has a new post to his blog today showing how you can use the Zend_Amf component in your Flex+PHP application to authenticate users with the same authentication structure the rest of the application uses.
I forget why, but a few days ago I started doing some digging around with authentication in Zend_Amf_Server. I had figured that I would add an adapter to the Zend_Amf_Server::setAuth() method and that would be it. But I was wrong.
Zend_Auth is used to validate the login information that's passed along and the Zend_Acl checks to see if they have access. To make this work for a connection from Flex, you need to make an authentication adapter and add it (as well as a basic ACL) into your Zend_Amf_Server call.