can't abe to use session varible in routes.php file in codeigniter?

[eluser]Unknown[/eluser]
i am use following code to retrieve the session varible in routes.php

if($this->db_session->userdata('request_url')!="")
{
$route['user/(:any)'] = "search_user_name/redirect_url/".$_SESSION['request_url'];
$this->db_session->unset_userdata('request_url');
}
else {
$route['user/(:any)'] = "search_user_name/index/$1";
}

the session varible would be set into template/header.php

$this->db_session->set_userdata('request_url',$_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"]);