UserController.php

<?php

namespace frontend\modules\user\controllers;

use common\base\MultiModel;
use frontend\models\search\OrderSearch;
use frontend\modules\user\models\AccountForm;
use frontend\modules\user\models\SignupOauthForm;
use Yii;
use yii\authclient\clients\Facebook;
use yii\authclient\clients\Google;
use yii\helpers\Url;
use yii\web\Controller;
use common\models\User;
use common\models\UserToken;
use frontend\modules\user\models\LoginForm;
use frontend\modules\user\models\PasswordResetRequestForm;
use frontend\modules\user\models\ResetPasswordForm;
use frontend\modules\user\models\SignupForm;
use yii\base\Exception;
use yii\base\InvalidParamException;
use yii\filters\AccessControl;
use yii\filters\VerbFilter;
use yii\helpers\ArrayHelper;
use yii\web\BadRequestHttpException;
use yii\web\NotFoundHttpException;
use yii\web\Response;

/**
 * Class UserController
 * @package frontend\modules\user\controllers
 */
class UserController extends Controller
{
  ...
    /**
     * @return array|string|Response
     */
    public function actionLogin()
    {
        $request = Yii::$app->request;
        $model = new LoginForm();
        if ($request->isAjax) {
            $model->setFormName('-modal');
        }

        $oldCartItems = Yii::$app->cart->items;

        if ($request->isPost
            && ($model->load($request->post()) || $model->load($request->post(), $model->formName() . '-modal'))
            && $model->login()) {

            $this->syncCartItems($oldCartItems);

            return $this->redirect($request->referrer);
        }
        if ($request->isAjax) {
            return $this->renderAjax('login', ['model' => $model]);
        }
        return $this->render('login', ['model' => $model]);
    }

  ...
    /**
     * @param $oldCartItems
     */
    protected function syncCartItems($oldCartItems){
        $cart = Yii::$app->cart;
        if($oldCartItems){
            $cart->storage = 'hscstudio\cart\DatabaseStorage';
            $cart->init();
            foreach ($oldCartItems as $item){
                $cart->create($item, $item->getQuantity());
            }
        }
    }
}