I was using the serializing approach between Flash and PHP for the longest time before AMFPHP had its 1.0 release (wow...that was a long time ago...in 2006)!
Serializing using a sepiroth's AS library paired with PHP's built in serializing functions worked and I didn't change it because it did the job.
Should I switch over to the AMF format and technology? One pro is that the data exchange is probably more efficient for AMF because it's a binary format.
I've also see other scripting languages/frameworks such as Python/Django, Ruby/Rails supporting AMF. I guess AMF is also a standard format. Does each language serialize data differently?