Method getCategoryChildrenString

public static function getCategoryChildrenString($id, $children = '')
{
    $db = Factory::getDBO();
    $query = "SELECT id FROM #__phocacart_categories WHERE parent_id = " . (int) $id;
    $db->setQuery($query);
    $categories = $db->loadColumn();
    if (!empty($categories)) {
        foreach ($categories as $k => $v) {
            if ($children != '') {
                $children .= ',';
            }
            $children .= $v;
            $children = self::getCategoryChildrenString($v, $children);
        }
    }
    return $children;
}