Дерево из элементов и разделов

 /***********
 * Разделы *
 ***********/

$rs_section = CIBlockSection::GetList(Array("LEFT_MARGIN" => "ASC"), $arFilter, true, Array("UF_*"));
while($ar_section = $rs_section->Fetch())
{
   $arResult["SECTIONS"][$ar_section["ID"]] = $ar_section;
}

/************
 * Элементы *
 ************/

$rs_element = CIBlockElement::GetList(Array(), $arFilter, false, false, $arSelect);
while($ar_element = $rs_element->GetNext(false, false))
{   
   if(array_key_exists($ar_element["IBLOCK_SECTION_ID"], $arResult["ELEMENTS"]))
   {
      $arResult["ELEMENTS"][$ar_element["IBLOCK_SECTION_ID"]][$ar_element["ID"]] = $ar_element;
   }
   else
   {
      $arResult["ELEMENTS"][$ar_element["IBLOCK_SECTION_ID"]] = Array();
      $arResult["ELEMENTS"][$ar_element["IBLOCK_SECTION_ID"]][$ar_element["ID"]] = $ar_element;
   }   
}

/*********************
 * Построение дерева *
 *********************/

$previousLevel = 0;
$arStack = Array();
foreach($arResult["SECTIONS"] as $arSection)
{
   if($previousLevel && $arSection["DEPTH_LEVEL"] <= $previousLevel)
   {      
      $delta = $previousLevel - $arSection["DEPTH_LEVEL"];
      for($i = 0; $i < ($delta + 1); $i++)
      {
         $section_id = array_pop($arStack);
         if(array_key_exists($section_id, $arResult["ELEMENTS"]))
         {   
            foreach($arResult["ELEMENTS"][$section_id] as $arElement)
            {
               $arResult["ITEMS"][] = $arElement;
            }
         }
      }
   }

   $arResult["ITEMS"][] = $arSection;   
   
   $arStack[] = $arSection["ID"];   
   $previousLevel = $arSection["DEPTH_LEVEL"];   
}