Add real time collaboration to PHP example

[mostlyjason]

I'd like to request that you add real time collaboration to the PHP example. I'm very interested in seeing it all tied together with real-time collaboration, track, changes, comments, and the adapter integrations.

[pjasiun]

Real-time collaboration is far more complicated than comments and track-changes features. Also, the integration of the track changes and comments features with real-time collaboration is very ticky. This is why it requires CKEditor Cloud Services backend to run it. We decided to provide ready to use backend (as a cloud and as an on-premise solution) instead of asking developers to create their custom backend. This is why it is not added to the PHP sample which shows how to integrate a custom backend.