To solve the problem that pressing the browser back button to return, after closing session not will start again without asking for data entry, add the following code.
$this->output->set_header('Last-Modified:' . gmdate('D, d M Y H:i:s') . 'GMT');
$this->output->set_header('Cache-Control: no-store, no-cache, must-revalidate');
$this->output->set_header('Cache-Control: post-check=0, pre-check=0', false);
$this->output->set_header('Pragma: no-cache');
But when I press the back button of the browser I get the message Confirm Form Resubmission. sorry my bad English