pass php session identifier between frontend and backend pages

I have a website based completely on php pages for frontend and backend and I use session to store some info. This way I just need to put session_start() on top of each page to have the session info available on page.
I would like to move to a different architecture and maintain backend on php while having frontend on html.

With this architecture how can I manage to have the session shared between the backend pages?
As per the documentation:

session_start() creates a session or resumes the current one based on a session identifier passed via a GET or POST request, or passed via a cookie

so I imagine I will have to pass somehow the session identifier back and forth between back and front end. Am I correct? If yes, how can I do it?