Pimcore\Model\Element\Service::getIdList PHP Method

    public static function getIdList($list, $idGetter = 'getId')
    {
        $ids = [];
        if (is_array($list)) {
            foreach ($list as $entry) {
                if (is_object($entry) && method_exists($entry, $idGetter)) {
                    $ids[] = $entry->{$idGetter}();
                } elseif (is_scalar($entry)) {
                    $ids[] = $entry;
                }
            }
        }
        if ($list instanceof \Pimcore\Model\Listing\AbstractListing) {
            $ids = $list->loadIdList();
        }
        $ids = array_unique($ids);
        return $ids;
    }