yii2 session handling

use  yii\web\Session;

$session = Yii::$app->session;

// start session
$session->open();

// close session
$session->close();

// destroys session.
$session->destroy();

//store value in session
$session['user_id'] = 2;

echo $session['user_id'];
//result will be 2

// check if a session is set
if ($session->isActive)

//unset session variable
unset($_SESSION['user_id']);