$customer = new Customer(№ клиента);
if (Validate::isLoadedObject($customer))
{
    $context = Context::getContext();
    $context->cookie->id_customer = (int)$customer->id;
    $context->cookie->customer_firstname = $customer->firstname;
    $context->cookie->customer_lastname = $customer->lastname;
    $context->cookie->is_guest = $customer->isGuest();
    $context->cookie->passwd = $customer->passwd;
    $context->cookie->email = $customer->email;
    $context->cookie->logged = true;
    $context->cookie->write();
}

$this->login_customer(17);

$sql = "SELECT * FROM `" . _DB_PREFIX_ . "customer` WHERE `id_customer` = '".$id_customer."'";
$result = Db::getInstance ()->GetRow ($sql);

$id = Db::getInstance ()->getValue('SELECT `id_customer` FROM `' . _DB_PREFIX_ . 'customer` WHERE `id_customer` = ' . (int)$id_customer)

$id = Db::getInstance ()->getValue('SELECT `id_customer` FROM `' . _DB_PREFIX_ . 'customer` WHERE `id_customer` = "'.(int)$id_customer.'"');