How to store class variable inside a Craft controller namespace Craft;

class Test_FormsController extends BaseController
{

   protected $allowAnonymous = true;
   public $session_id;

    public function actionApply(){
        $appl_template = "1";
        $this-&gt;session_id = 15558145;
        return $this-&gt;renderTemplate("apply/step".$appl_template.".html");
    }

    public function actionSaveDataStruct(){
        echo 'session:'.$this-&gt;session_id.'&lt;br /&gt;';
        exit();
    }
}


Hi guys i'm newbie on Craft and my question is:

acctionApply loads form which will send to actionSaveDataStruct, I set a class variable $session_id on actionApply but whenever the form submitted and goes to actionSaveDataStruct, $this-&gt;session_id become null again, how do I set variable that will never reset whenever the page load?
In addition to passing the value along through a cookie, you could also do it through PHP's session support.

Adding:

craft()-&gt;httpSession-&gt;add('sessionId', '123456');


Retrieving:

$sessionId = craft()-&gt;httpSession-&gt;get('sessionId');


Deleting:

craft()-&gt;httpSession-&gt;remove('sessionId');