What if the session is needed immediately?

Question

PHP

session_start(1800);
var_dump($_SERVER["HTTP_COOKIE"]);

Answer:

Notice: Undefined index: HTTP_COOKIE

because the session has started, but the $_SERVER["HTTP_COOKIE"] variable will only be there when the page is updated or when moving to the next page, but I need a session the first time the visitor enters the first (any) page of the site.
How can you get out of this situation?

Answer 1

what is the connection between the start of the session and the withdrawal of the cookie?
session is available immediately after the start.