会话存储Codeigniter中的问题

Question

I got this issue when trying to store data to $_SESSION variable in Codeigniter 3.03

    $search = $_SESSION['search_result']['outbound'];

    $flight = array();
    foreach ($search as $item) {
        if ($item['id'] == $id) {
            $flight = $item;
            break;
        }
    }

    var_dump($flight);

    $_SESSION['selected_outbound'] = $flight;

The var_dump function return an unempty array, but when viewing session variable, it empty. Here is my session setting in application/config/config.php

$config['sess_driver'] = 'files';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = BASE . '/ci_sessions'; //BASE is root folder
$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = FALSE;

Instead of using $_SESSION directly, I also try to use $this->session->set_userdata('key', $variable) but no lucky.

Autoloading session class has been set up.

What am I wrong in this case ? Thanks