<?php
require($_SERVER["DOCUMENT_ROOT"] . "/bitrix/modules/main/include/prolog_before.php");
?>
<?
CModule::IncludeModule("iblock");
CModule::IncludeModule("sale");
CModule::IncludeModule("catalog");

$productId =$_REQUEST["id"];
$quantity = $_REQUEST["count"];

$basket = \Bitrix\Sale\Basket::loadItemsForFUser(
    \Bitrix\Sale\Fuser::getId(),
    \Bitrix\Main\Context::getCurrent()->getSite()
);

if ($item = $basket->getExistsItem('catalog', $productId)){

    $item->setField('QUANTITY', $item->getQuantity() + $quantity);
}else{

    $item = $basket->createItem('catalog', $productId);
    $item->setFields([
        'QUANTITY' => $quantity,
        'CURRENCY' => \Bitrix\Currency\CurrencyManager::getBaseCurrency(),
        'LID' => \Bitrix\Main\Context::getCurrent()->getSite(),
        'PRODUCT_PROVIDER_CLASS' => 'CCatalogProductProvider',
    ]);
}

$basket->save();
?>
<? require($_SERVER["DOCUMENT_ROOT"] . "/bitrix/modules/main/include/epilog_after.php");
?>