sess_expire_on_close

Hello everyone...
I want to override or set config item dynamically. If user logged in without checking remember me checkbox then I want to set sess_expire_on_close value is false and user not checked then sess_expire_on_close value is true. I used the following php code: 

PHP Code:

if($this->input->post('remember')) 
$this->config->set_item('sess_expire_on_close', '0');
else
$this->config->set_item('sess_expire_on_close', '1'); 


but this process is not working. When I closed then browser without checking remember me dialog then session expire value is set to be 0 but still session is live. If a set config the following value 

PHP Code:

$config['sess_expire_on_close'] = TRUE; --> config.php 


then session will be destroy when closed browser but not store session value when I checked remember me checkbox.