Can somebody throw me in the right direction in order to get emails forwarded from Google Apps to be piped by Postfix to a PHP Script? This answer didn't helped much but to tell me that I need to confgiure a subdomain in order to receive the email and pipe them to a php script. Is it right?
You will need a MX record pointing at the postfix server, for example:
host.example.com IN MX postfixserver.example.com
Then you can set up forwarding to forward to email@example.com where address is the alias on the postfix server that's piping to the script.
Also, be aware, only system aliases in /etc/aliases can be piped to scripts in Postfix, virtual aliases can't be setup to use a pipe.