$items_sel =  (new CIBlockElement())->GetList([], $filter, 0, 0, ['ID']);
$arResult['ITEMS_SELECTED'] = $items_sel->SelectedRowsCount();

if(!$arResult['ITEMS_SELECTED'])
    $ids = [];
else
    while($item = $items_sel->Fetch())
        $ids[] = $item['ID'];