How to understand concepts of php easily? I am ok with html css

Answer 1

Learn PHP:

https://www.learn-php.org/

https://www.w3schools.com/php/DEFAULT.asp

https://www.sololearn.com/Course/PHP/?ref=app

Answer 2

I got that:
implementation of code in to variables is a power of php
Since i didnt code in php for last 12 years - i cant say you more