public function addToWishlist($post_id, $sku, $item_option = [], $advance = '') {
        $exists = $this->guards->itemExists($post_id, $sku);
        if ($exists instanceof GenericError) {
            return $exists;
        }

        if (!defined('WCEX_WIDGET_CART')) {
            if (!empty($item_option)) {
                // by default we use `wcex_widget_cart`'s way of serializing option data but multi-select
                // option values are serialized differently when not using `wcex_widget_cart`. Because of this,
                // we have to transform multi-select option values when `wcex_widget_cart` is not being used.
                $item_option = self::convertWidgetCartItemOptionPayloadToWelcartDefault($item_option, $post_id, $sku);
            }

            // By default, Welcart serializes with the encoded SKU. `wcex_widget_cart` serializes with the decoded SKU.
            $sku = urlencode($sku);
        }

        $opterror = $this->guards->requiredOptionsProvided($post_id, $sku, $item_option);
        if ($opterror instanceof GenericError) {
            return $opterror;
        }

        $tempcart = new \usces_cart();
        if (!empty($item_option)) {
            // we are forced to set $_POST at runtime here since `in_serialize` uses $_POST internally
            $_POST['itemOption'] = $item_option;
        }
        $tempcart->in_serialize($post_id, $sku);
        unset($_POST['itemOption']);
        if (!empty($advance)) {
            $advance = $tempcart->wc_serialize($advance);
        }

        $hookargs = [
            'post_id' => $post_id,
            'sku' => $sku,
            'item_option' => $item_option,
            'advance' => $advance,
        ];

        $customerror = apply_filters('wcexwl_filter_item_add_to_wishlist_custom_validation', null, $hookargs);
        if ($customerror !== null) {
            return $customerror;
        }

        $serial = apply_filters('wcexwl_filter_item_add_to_wishlist_serial', $tempcart->serial, $hookargs);

        // check if logged in after all other checks pass so that we can add the item
        // to the wishlist automatically after a successful login
        $isloggedin = $this->guards->isLoggedIn();
        if ($isloggedin instanceof GenericError) {
            $isloggedin->setData($serial);
            return $isloggedin;
        }

        $res = CRUD::executeAddToWishlist($post_id, $sku, $serial, $advance);
        if ($res === false) {
            return $this->master->estore->getErrorResponse(Store::INTERNAL_SERVER_ERROR);
        }

        return true;
    }