public static function addComboSetCells($line_sku, $sku) {
        $td_h = ',"';
        $td_f = '"';

        $skumetaid = $sku['meta_id'];
        $comboset = ComboSet::getComboSetBySkuMetaId($skumetaid);
        if ($comboset instanceof GenericError) {
            for ($i = 0; $i < count(self::getHeaders()); $i++) {
                $line_sku .= $td_h . $td_f;
            }

            return $line_sku;
        }

        $jsona = $comboset->getJson();
        $jsona['groups'] = [];
        foreach ($comboset->getGroups() as $group) {
            $groupjson = $group->jsonSerialize();
            $items = [];
            foreach ($group->getItems() as $item) {
                $sku = wel_get_sku_by_id($item->getSkuMetaId());
                $ijson = $item->getJson();
                $ijson['skucode'] = $sku['code'];
                $ijson['postId'] = $item->getPostId();
                $items[] = $ijson;
            }
            $groupjson['items'] = $items;
            $jsona['groups'][] = $groupjson;
        }

        // output up to 5MB is kept in memory, if it becomes bigger it will automatically be written to a temporary file
        $csv = fopen('php://temp/maxmemory:' . (5 * 1024 * 1024), 'r+');

        fputcsv($csv, [json_encode($jsona)]);

        rewind($csv);

        // put it all in a variable
        $output = stream_get_contents($csv);

        $line_sku .= ',' . trim($output);

        return $line_sku;
    }