<?
//проверяем на активность
$cur_page = $APPLICATION->GetCurPage(false);
$is_active = ($arItem['SECTION_PAGE_URL'] == $cur_page || strpos($cur_page,$arItem['SECTION_PAGE_URL'])!==false)?" active":"";
?>
<a
href="<?= $arItem['SECTION_PAGE_URL'] ?>"
class="button button--underline catalog-aside-list-item__link<?if($is_active):?> active<?endif;?>"
>
<?= $arItem['NAME'] ?>
</a>