__construct passing data?

$data is a local variable of your constructor. You need to make a member variable of your controller class if you want to use it somewhere else in other functions of your controller.

PHP Code:

class Staff extends MY_Controller {

    private $data;

    public function __construct()
    {
        parent::__construct();
        $this->load->model('staff_model');
        $this->data['session'] = $this->get_session_information();
    }

    public function index()
    {
        $this->load->view('hello', $this->data);
    }