How to synchronize a fight between two or more players?

Question

Game development

The bottom line is this - a tower defense style game online, presumably on android (although it seems to me that it doesn't matter), one player builds a defense and the other then attacks.
It was decided that the attacker should be online, the scene of the defender is loaded for him.
How to proceed, I mean that she both saw the same thing?
Should one fight take place and the other observe? while the data is stored on the server and issued to another.
Or how is it done right?
If possible, examples of code implementation or an article where this is described ...
And is it possible to do the server part in PHP or use C for such cases ...

In general, I would like to discuss this topic, how to implement this.

Answer 1

Well, look, for example, towards ready-made solutions - the same Photon.

Answer 2

Technology - websocket.
Language doesn't matter.