Q:

AMFPHP calling a function in FLEX

is it possible to call a function in flex from the php side of AMFPHP?
For instance, and error is thrown in php, can i "listen" for it in the flex app?

A:

is it possible to call a function in flex from the php side of AMFPHP?

No; AMFPHP operates on the request response model.  So, the client must request something from a server.  The server cannot push information to the client.  
To push data from the server to the client you need a server that supports it.  LiveCycle DS is one option.  GraniteDS is another.  

