How to make the PHP know the real SCRIPT_FILENAME?

I am trying to use PHP as backend, which communicate with apache-2.3's mod_proxy_fcgi But I have noticed that the SCRIPT_FILENAME has been changed to "proxy:balancer://xxxxxx", it cann't be recognized by the remote PHP backend. So the request failed. In my opinion, the remote PHP backend use the SCRIPT_FILENAME to find the script file. But the PHP and the Apache may be in different computer, so the Document Root may be different. is that means the PHP shouldn't rely on the SCRIPT_FILENAME? how can I configure the PHP to know the real script file to execute? Thank you