I had the hardest time getting AMFphp to work. Not much in terms of documentation or forum activity.
AMFPHP.org last update 10.19.2008, doesn't look like that project is alive anymore.
ZendAMF looks interesting because it's actually integrated into Flash Builder.
I currently use WebORB for all my projects.
Examples are enough to get you going
Documentation is okay, they even have a couple of videos.
Code Generation is helpful
Data Introspection is a life saver.
I have used AMFPHP in the past and was very happy with it, but seeing as the latest version is rather old (as you noticed as well) I got me thinking.
I was wondering if ZendAMF was anygood, it looks the most appealing to me from the list of names (reputation-wise).