如何在Codeigniter pre_system挂钩中使用Sessions

I am trying to change some config variables depends on SESSION data. I use session_start and set some variables in session, it works normally. But when I logout system and clear sessions, codeigniter doesn't clear and destroy the data in session.

Ci_session cleared but PHPSESSID doesn't change value, I think codeigniter uses PHPSESSID and again set all session values. Here is the screenshot: