Project: Connecting Search Bar with Database

I am a beginner and working on connecting a search bar with a database.
It is all pretty new for me and i am learning throw watching youtube videos.

I have question can i link a .PHP File like a .CSS File into my html.index File?
If not how do i connect a .php File so it affects the searchbar (input) on my html.index file?

Client <======> Server

Javascript <========> Php

Php <===========> Database

You use ajax to communicate from javascript to php.
You would need to use the value of the searchbar to php.

Then on php side,. you would do a select in database using a query like so:
SELECT * FROM accounts WHERE name like ‘%’ . $searchString . ‘% LIMIT 5’;

You will want to escape $searchString of any potential sql injection code. If you’re a total beginner this is quite a daunting task because it requires knowing 3 different languages and knowing how to connect to database with server., which implies you’ve already installed a database… preferably MYSQL.

None of this has anything to do with html/css.