Hi,
That should not be to hard to configure. By default, the Cisco ACS server records stop/start accounting records for both TACACS+ and RADIUS. This is logged in comma separated csv files in the logging directory (configurable).
With PHP it should not be hard to read comma separated files I guess.
The challenge though is how to link to them, I would say create a share on the server that runs the (to be build) billing application and configure ACS to used the share as logging directory. That way you don't have to worry about disk space at the ACS server, and you could build cleaning methods in the application as well.
Don't have experience in PHP either, but this would be my general approach :-)
Let me know if this helps,
Leo