How to properly build the application structure on the backend (good practice)?

Question

PHP

Hello!
There is a web application, at the moment it has 3 databases, access to which is needed from several parts of this application. Interested in good practice, how to properly structure the project so that there is no code repetition and scalability and flexibility remain)
Outline of the current state of affairs in the project

There is a certain admin panel through which data is received and written to the database, either directly through it or through the main part of the application.
There is an idea to do everything through the API and just make requests to it.

What could be the disadvantages of this approach?
Tell me where to dig or what to read?)

Answer 1

I am for the second option - it is more flexible and you separate the front from the back completely.