To establish a audio/video call WeRTC need to send some information ( SDP and candidate ) from one client to other client
this is done by signalling. Browser can not send this formation directly to the other Browser.
Browser need to communicate with a server application to do this signalling. This can be done by Websocket or normal html ( like BOSH )
For signalling you need to use standalone application ( not CGI) in the server side.
This can be developed using C/C++ , Java script using node.js or PHP.
and for client side you can use only JavaScript in the browser.
For signalling you need to understand how server can communicate with the browser.
Normally server can not send anything until browser makes a request
This can be overcome by using Websocket or http polling ( like BOSH )
Most of the WebRTC example use node.js server and JavaScript but it can be developed using PHP.
You need to implement Webscoket server or BOSH server using PHP to send and receive from client.
Then modify the messages according to your requirements.
I think there is no example available on github. Maybe PHP is not very popular for standalone application.