PHP & Unity - Moving Info back and forth

I posted not long ago and decided to make another thread.

In our application we ask the players to create a new account, when they do we add this information to our DB.

The next step is to now grab the account they created, so that we can add more data to it. So the player creates an account with us, after this is done we then login with Google Play Services, and we now need to add info from google to the account they created, locale, id, etc

My problem here being new to php is im not sure on the correct way to do this so im asking for some guidance on it and have a couple of questions that may or may not make any sense.

1. When a user creates an account i now need to reference and keep a hold of this account they created so that we can easily access and edit/add info to it. How would you accomplish this?

2. When i need information from the database to display in the application how would i grab this info specific to the player and read it directly into unity? I think i could use the JsonEncode but im not sure if this is the most efficient way to do this.

There is more but i don't want to ask to much all at once haha.

Thanks for any and all help.

John