creating php rest api from scratch with no framework

think of yourself at sitting at a table at a restaurant in your one piece of software such as a web application on the client side and then the kitchen is the server or the service that processes certain requests well the waiter is like an API is formatted to take a certain order in a specific way and then bring back a certain meal this would be the data or the response that you requested so an API is basically a messenger or a waiter between running software

example: login/register using [google, fb, apple,...] and other companies that provide api for authentication.
if you have an gmail(goggle) account you can use that to register in our website which creates your own account in our network

https://www.youtube.com/playlist?list=PLillGF-RfqbZ3_Xr8do7Q2R752xYrDRAo https://github.com/bradtraversy/php_rest_myblog