Starting with Php and Js [on hold]

I'm using a bootstrap starting project (You can get some here) to learn more about web development.

I'm a little stuck about how to use Php in this project to make a small back-end and make a connection to save some data in the database.

Bellow is a screenshot of the structure of my project.

Project stucture

How can I start adding php to my project? What do I need to do that?

Obs: I'm also new to php.

edit: I was thinking about creating a folder to put only php files and call these files on Js when needed to access the database. Is it possible?