When is sessionid triggered?

When is sessionid generated? When a person opens this page, what will happen? & Nbsp; do you still need to wait until the backend logic contains & nbsp; session_start & nbsp; or & nbsp; operator _ SESSION [XXX ?, When is sessi sessionid generated?
When a person opens this page, what will happen?

Or will it be generated after session_start or _ SESSION [XXX] is in your background logic?
------ Solution --------------------
When the session_start function is executed
Before session_start (), you can use session_id ($ sid) to specify the sessionid you need.