Private api or how to do it right?

Question

PHP

I am making a server part for storing data, I will receive data from android. For example, there is a register.php file in which a user is created (POST request) and recorded in a certain table ... There is also a certain file in which (GET request) and we get, for example, user data, his password and other personal information, how can I do this so that others do not have access to this request. Maybe I don't understand what direction to go in..

Answer 1

And in what a problem to use authorization and sessions?