How to use session identifier for resuming an existing session with session_start?

Question

php.net states:

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.

There are some discussions on setting/changing the session id or the right usage of session_start (here, here, here, here or here).

I want to make sure that the user's session is re-used, so I would think that I need to pass the Session-Id. But it seems that just calling session_start is enough for re-identifying the user (and hers authentication).

This code would be fine?

// helper:
<?php
    function isUserAuth(){
        if(session_status() === PHP_SESSION_ACTIVE){
            if ($_SESSION['auth'] === 1) { // set in user-login.php on success
                return true;
            }
        }
        return false;
    }
?>

// some endpoint
<?php
    session_start();
    if (!isUserAuth()) {
        header('HTTP/1.0 403 Forbidden');
        exit(getSingleMsgJson('api:error:noSession'));
    }

    // or do things here: user is authenticated
?>

Would setting the Session Id with the original Session Id from the login change anything here? How would I resumes the current [session] based on a session identifier passed via a GET or POST exactly?