Session ID

Hi im a newbbie , i need you help or advise , 

im making some report and i need to use the "id" from 

PHP Code:

$this->session->userdata('logged_in'); 


 i check 
$ses= $this->session->userdata('logged_in');
echo '<pre>';
print_r($ses);
and my data is 
[id] => 23
   [username] =>John Doe 

now how i can use this id because i do this and dont show anything 

$ses= $this->session->userdata('id');

echo '<pre>';
print_r($ses);