src/web/User.php
protected function sendIdentityCookie($identity, $duration)
    {
        $cookie = new Cookie($this->identityCookie);
        $cookie->value = json_encode([
            $identity->getId(),
            $identity->getAuthKey(),
            $duration,
        ], JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
        $cookie->expire = time() + $duration;
        Yii::$app->getResponse()->getCookies()->add($cookie);
    }
protected function sendIdentityCookie($identity, $duration)
    {
        $cookie = new Cookie($this->identityCookie);
        $cookie->value = json_encode([
            $identity->getId(),
            $identity->getAuthKey(),
            $duration,
        ], JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE);
        $cookie->expire = time() + $duration;
        Yii::$app->getResponse()->getCookies()->add($cookie);
    }
Sends an identity cookie.
This method is used when [[enableAutoLogin]] is true.
It saves [[id]], [[IdentityInterface::getAuthKey()|auth key]], and the duration of cookie-based login
information in the cookie.
@param IdentityInterface $identity
@param integer $duration number of seconds that the user can remain in logged-in status.
@see loginByCookie()