CIntranetUtils::GetIBlockSectionChildren

static function GetIBlockSectionChildren($arSections)
{
	if (!is_array($arSections))
	{
		$arSections = [ $arSections ];
	}

	$dbRes = CIBlockSection::GetList(
		[ 'LEFT_MARGIN' => 'asc' ],
		[ 'ID' => $arSections ],
		false,
		[ 'ID', 'IBLOCK_ID', 'LEFT_MARGIN', 'RIGHT_MARGIN' ]
	);

	$arChildren = [];
	while ($arSection = $dbRes->Fetch())
	{
		if (
			$arSection['RIGHT_MARGIN'] - $arSection['LEFT_MARGIN'] > 1
			&& !in_array((int)$arSection['ID'], $arChildren, true)
		)
		{
			$dbChildren = CIBlockSection::GetList(
				[ 'id' => 'asc' ],
				[
					'IBLOCK_ID' => $arSection['IBLOCK_ID'],
					'ACTIVE' => 'Y',
					'>LEFT_BORDER' => $arSection['LEFT_MARGIN'],
					'$arSection['RIGHT_MARGIN'],
				],
				false,
				[ 'ID' ]
			);

			while ($arChild = $dbChildren->Fetch())
			{
				$arChildren[] = (int)$arChild['ID'];
			}
		}
	}

	return array_unique(array_merge($arSections, $arChildren));
}