use  yii\web\Session;
 
$session = Yii::$app->session;
// check if a session is already open
if ($session->isActive) ...
// open a session
$session->open();
// close a session
$session->close();
// destroys all data registered to a session.
$session->destroy();

$session = Yii::$app->session;
$session->set('user_id', '1234');
//OR
$session['user_id'] = '1234';
//OR
$_SESSION['user_id'] = '1234';

$session = Yii::$app->session;
$user_id = $session->get('user_id');
//OR
$user_id = $session['user_id'];
//OR
$user_id = isset($_SESSION['user_id']) ? $_SESSION['user_id'] : null;

$session = Yii::$app->session;
$session->remove('user_id');
//OR
unset($session['user_id']);
//OR
unset($_SESSION['user_id']);