create_sid() is called when a new session id is needed.
Such as:
0. With PHP's default session handler, when session.use_strict_mode is turned on, if a session id provided by the client doesn't exist on the server, create_sid() is called to generate a new session id.
1. When validateId() is provided and it returns false, create_sid() is called to generate a new session id.
2. When session_regenerate_id() is called, create_sid() is called to generate a new session id.