Bool in session is turned into string

[eluser]überfuzz[/eluser]
I did a login system, quite simple but its dandy. But looking through the code I found something puzzling.

In the function setting the session-value to state logged in.

Code:

$data = array(
    'username' => $this->input->post('username'),
    //etc...
    'logged_in' => TRUE //this would be the boolean.
);
$this->session->set_userdata($data);

In the my MY_Controller I only just realized that I was checking logged_in like this:

Code:

if($this->session->userdata('logged_in') != TRUE ) //not !==
{
    redirect('login');
}

!== didn't work so I trowed a

Code:

var_dump($this->session->userdata('logged_in')); //output string(1) "1"

Why is the bool turned into a string?

Edit, a glitch pointed out by bigtony plus != TRUE