How to check if a user is logged in?

Question

PHP

I have a server on which I created a subdomain ooo.domain.comand set up redirection of requests to another server through DNS

Now the task is to check whether the user is authorized or not in order to understand what content to display.

On the main domain I check like this:

if (isset($_SESSION["user"]) && !empty($_SESSION["user"]))

Question: How can I check if a user is authorized on the main domain?