Folder::getSubFolderIds

static function getSubFolderIds(int $folderId): array
{
	$ids = [];

	$res = self::getList([
		'select' => ['ID'],
		'filter' => [
			'PARENT_ID' => $folderId,
		],
	]);
	while ($row = $res->fetch())
	{
		array_push($ids, $row['ID'], ...self::getSubFolderIds($row['ID']));
	}

	return $ids;
}