$rsParentSection = CIBlockSection::GetList(
	Array('name' => 'asc'),
	Array('IBLOCK_ID' => 21, 'ACTIVE' => 'Y')
);

while ($arParentSection = $rsParentSection->GetNext()) {

    $arrFullListSection[] = $arParentSection["ID"];

	$arFilter = array(
        'IBLOCK_ID' => $arParentSection['IBLOCK_ID'],
        '>LEFT_MARGIN' => $arParentSection['LEFT_MARGIN'],
        ' $arParentSection['RIGHT_MARGIN'],
        '>DEPTH_LEVEL' => $arParentSection['DEPTH_LEVEL']
    );

    /* делаем запрос внутри запроса по id подраздела */
	$rsSect = CIBlockSection::GetList(array('left_margin' => 'asc'), $arFilter);

	while ($arSect = $rsSect->GetNext()){
        $arrFullListSection[] = $arSect["ID"];
	}
}

echo '
';
var_dump($arrFullListSection);
echo '
';

Получить все id разделов и подразделов каталога по id инфоблока. Битрикс

<p>Получить все id разделов и подразделов каталога по id инфоблока</p> <pre><code class="language-php language-html"> $rsParentSection = CIBlockSection::GetList( Array('name' =&gt; 'asc'), Array('IBLOCK_ID' =&gt; 21, 'ACTIVE' =&gt; 'Y') ); while ($arParentSection = $rsParentSection-&gt;GetNext()) { $arrFullListSection[] = $arParentSection["ID"]; $arFilter = array( 'IBLOCK_ID' =&gt; $arParentSection['IBLOCK_ID'], '&gt;LEFT_MARGIN' =&gt; $arParentSection['LEFT_MARGIN'], '<span> $arParentSection['RIGHT_MARGIN'], '&gt;DEPTH_LEVEL' =&gt; $arParentSection['DEPTH_LEVEL'] ); /* делаем запрос внутри запроса по id подраздела */ $rsSect = CIBlockSection::GetList(array('left_margin' =&gt; 'asc'), $arFilter); while ($arSect = $rsSect-&gt;GetNext()){ $arrFullListSection[] = $arSect["ID"]; } } echo '<pre>'; var_dump($arrFullListSection); echo '</pre>'; </span></code></pre>