$session = Yii::$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();

Cookies : Yii represents each cookie as an object of yii\web\Cookie. Both yii\web\Request and yii\web\Response maintain a collection of cookies via the property named cookies. The cookie collection in the former represents the cookies submitted in a request, while the cookie collection in the latter represents the cookies that are to be sent to the user.

The part of the application dealing with request and response directly is controller. Therefore, cookies should be read and sent in controller.

Reading Cookies : You can get the cookies in the current request using the following code:

// get the cookie collection (yii\web\CookieCollection) from the "request" component

$cookies = Yii::$app->request->cookies;



// get the "language" cookie value. If the cookie does not exist, return "en" as the default value.

$language = $cookies->getValue('language', 'en');



// an alternative way of getting the "language" cookie value

if (($cookie = $cookies->get('language')) !== null) {

    $language = $cookie->value;

}



// you may also use $cookies like an array

if (isset($cookies['language'])) {

    $language = $cookies['language']->value;

}



// check if there is a "language" cookie

if ($cookies->has('language')) ...

if (isset($cookies['language'])) ...

Sending Cookies : You can send cookies to end users using the following code : 

// get the cookie collection (yii\web\CookieCollection) from the "response" component

$cookies = Yii::$app->response->cookies;



// add a new cookie to the response to be sent

$cookies->add(new \yii\web\Cookie([

    'name' => 'language',

    'value' => 'zh-CN',

]));



// remove a cookie

$cookies->remove('language');

// equivalent to the following

unset($cookies['language']);