I tested with FF and chrome.
I loged in and loged out.
Then when I click "Go Back" button, I can go to admin still.
It should ask to log in again.
I have the following codes in all the files in the admin folder as you suggested.
Code:
...
...
parent::Controller();
session_start();
if (isset($_SESSION['userid']) && $_SESSION['userid'] < 1){
redirect('welcome/verify','refresh');
}