会话Codeigniter

I have a problem with session in CI

When I do:

if no exists:
$_SESSION['test'] = 0;
if exist:
$foo = $_SESSION['test'];
$_SESSION['test'] = ++$foo;
echo $_SESSION['test'];

And it works perfect. Autoincrementing every load

But, if I do:

if no exist:
$this->session->set_userdata('test', 0);
if exists:
$s = $this->session->userdata('test');
$s++;
$this->session->set_userdata('test', $s);
var_dump($this->session->userdata('test'));

it doesn't work...

What I missed?

Thanks!

Idk what is the problem.

I tried: import library session session_start();

but nothing work. So I do it with $_SESSION[].

If anyone has any solution, welcome.

Thanks everybody