CI 4.3.5 - Session Expiration Not Working

Hi,

I am using CI 4.3.5
I have set

Code:

    /**
    * --------------------------------------------------------------------------
    * Session Expiration
    * --------------------------------------------------------------------------
    *
    * The number of SECONDS you want the session to last.
    * Setting to 0 (zero) means expire when the browser is closed.
    */
    public int $expiration = 43200;  // 12 hour - i need after 12 hour , session should be destroy for all users , so they can re login again.

But its not working  , can any one tell me what will be issue ?

Thanks