A personal server for my multiplayer using MySQL

Hi, since I do not have so much experience with databases, I would like to ask the community more on this matter. I and my friend are developing a game together which is going to be mainly based on a multiplayer architecture. Therefore, at this moment I am making use of a third party asset called Photon which is great for making a multiplayer game. I would totally recommend this asset to anyone, but my friend has recently suggested me an idea which might just be interesting and hard to implement. So far, we have made used of the server provided by the aforementioned third party, but my friend has a lot of experience with MySQL and would like to know whether it is possible to connect his server to my Unity game and make a functional multiplayer game.

I have made documentation on this matter and I have convinced myself that such thing is possible.Yet, all the amount of information found on the community is a bit encrypted, as a matter of saying.

I would like to be provided with a summary, I would like to know what course of action you would take in order to establish such an architecture (probably you have created one yourselves which is even better), do I need PHP functions and if yes, which are these, are there any tutorials or basic scripts that could show me how to make a basic connection?

Yes, there are a lot of questions, but should you give me a good base on this subject, I would go on understanding even more.