Create a Simple API OAuth 2.0 in PHP

I want the most simplest version of a PHP file that I can setup OAuth 2.0 in it so I can have the API works with security. I want to be able to send data from my site using this type of authentication. I wish to test the code using Postman with the token send from my site. I'm not an expert in this matter but I pretty much understand how it works. Please write ENTIENDO as the first word if you really understand what I want and know how to do it.