storage/DatabaseStorage.php
public function load(Cart $cart)
    {
        $items = [];
        $identifier = $this->getIdentifier(Yii::$app->session->getId());

        $query = new Query();
        $query->select($this->dataField)
            ->from($this->table)
            ->where([$this->idField => $identifier]);

        if ($data = $query->createCommand($this->_db)->queryScalar()) {
            $items = unserialize($data);
        }

        return $items;
    }
public function load(Cart $cart)
    {
        $items = [];
        $identifier = $this->getIdentifier(Yii::$app->session->getId());

        $query = new Query();
        $query->select($this->dataField)
            ->from($this->table)
            ->where([$this->idField => $identifier]);

        if ($data = $query->createCommand($this->_db)->queryScalar()) {
            $items = unserialize($data);
        }

        return $items;
    }
@param Cart $cart

@return mixed