AMFPHP 1.9 (yep, 1.9 -- although it's also called 2.0) that came out stable last 2nd of February 2010, I was almost ready to give up because of the "NetConnection.Call.BadVersion" errors that I'm getting when I try even a simple HelloWorld call. Followed every suggestions found on the Internet but none fixed the problem.
Then I tried checking out the AMFPHP code. I started with gateway.php. On line 106, changing PRODUCTION_SERVER to false fixed the problem.
It turns out that PRODUCTION_SERVER is set to true by default now.