How to retrieve session variable when the session had already started in another page?

How to retrieve session variable when the session had already started in
another page?

I had a question where tried to get the session variable that defined in
another page. But get error that the session had been started. when remove
the session_start(), it say undefined variable. Tried to search for other
relevant post but seems no clear solution given.. pls advise.. thanks..
session_start();
$CustID = $_SESSION['UserID'];