Вывод разделов и элементов для .left.menu_ext.php

<?php
if (!defined("B_PROLOG_INCLUDED") || B_PROLOG_INCLUDED !== true) die();

$IBLOCK_ID = 5;

if (!CModule::IncludeModule('iblock')) {
    return;
}

$aMenuLinks = [];

// Получение разделов
$allSections = [];
$rsSections = CIBlockSection::GetList(
    ['LEFT_MARGIN' => 'ASC'],
    ['IBLOCK_ID' => $IBLOCK_ID, 'ACTIVE' => 'Y'],
    false,
    ['ID', 'NAME', 'SECTION_PAGE_URL', 'DEPTH_LEVEL', 'IBLOCK_SECTION_ID']
);
while ($section = $rsSections->GetNext()) {
    $section['CHILDREN'] = [];
    $section['ELEMENTS'] = [];
    $allSections[$section['ID']] = $section;
}

// Построение дерева
$sectionTree = [];
foreach ($allSections as $id => $section) {
    $parentId = $section['IBLOCK_SECTION_ID'];
    if ($parentId > 0 && isset($allSections[$parentId])) {
        $allSections[$parentId]['CHILDREN'][$id] = &$allSections[$id];
    } else {
        $sectionTree[$id] = &$allSections[$id];
    }
}

// Получение элементов
$rsElements = CIBlockElement::GetList(
    ['SORT' => 'ASC'],
    ['IBLOCK_ID' => $IBLOCK_ID, 'ACTIVE' => 'Y'],
    false,
    false,
    ['ID', 'NAME', 'DETAIL_PAGE_URL', 'IBLOCK_SECTION_ID']
);
while ($element = $rsElements->GetNext()) {
    $sectionId = $element['IBLOCK_SECTION_ID'];
    if (isset($allSections[$sectionId])) {
        $allSections[$sectionId]['ELEMENTS'][] = [
            'NAME' => $element['NAME'],
            'URL'  => $element['DETAIL_PAGE_URL']
        ];
    }
}

// Построение меню (с вложенными подпунктами)
function buildMenuTree($tree) {
    $menu = [];
    foreach ($tree as $section) {
        $subMenu = [];

        // Элементы раздела
        foreach ($section['ELEMENTS'] as $element) {
            $subMenu[] = [
                $element['NAME'],
                $element['URL'],
                [],
                ['DEPTH_LEVEL' => $section['DEPTH_LEVEL'] + 1],
                ''
            ];
        }

        // Подразделы
        if (!empty($section['CHILDREN'])) {
            $subMenu = array_merge($subMenu, buildMenuTree($section['CHILDREN']));
        }

        $menu[] = [
            $section['NAME'],
            $section['SECTION_PAGE_URL'],
            $subMenu,
            ['DEPTH_LEVEL' => $section['DEPTH_LEVEL']],
            ''
        ];
    }
    return $menu;
}

$aMenuLinks = buildMenuTree($sectionTree);
?>

$obCache = new CPHPCache();
$cacheTime = 3600; // 1 час
$cacheDir = '/menu_cache/iblock_' . $IBLOCK_ID;

if ($obCache->InitCache($cacheTime, '', $cacheDir)) {
    $aMenuLinks = $obCache->GetVars();
} else {
    if ($obCache->StartDataCache()) {
        // ... весь код построения меню (от получения разделов до buildMenuTree)
        $aMenuLinks = buildMenuTree($sectionTree);
        $obCache->EndDataCache($aMenuLinks);
    } else {
        // fallback, если кеш не удалось создать
        $aMenuLinks = [];
    }
}