Problem when creating new cart with web service

[files___]

Hi folks,

I'm dealing with a problem when creating new cart using web service. 

 

This is the code I use to create new cart:

$xml = $this->controller->get(array('url' => $this->url .'/api/carts?schema=blank'));
        
$xml->cart->id_customer = $id_customer;
$xml->cart->id_address_delivery = 0;
$xml->cart->id_address_invoice = 0;
$xml->cart->id_currency = 1;
$xml->cart->id_lang = 1;
$xml->cart->id_carrier = 0;

$xml->cart->associations->cart_rows->cart_row->id_product = $id_product;
$xml->cart->associations->cart_rows->cart_row->quantity = 1;

if(!empty(Tools::getValue('product_attr')))
    $xml->cart->associations->cart_rows->cart_row->id_product_attribute = Tools::getValue('product_attr');

$opt = array('resource' => 'carts');
$opt['postXml'] = $xml->asXML();
$xml = $this->controller->add($opt);

return $xml->cart->id;

Practically, if I have the user logged, sometimes this code create new cart. (I can see the new cart in the Backoffice in the cart's list) but the front office prestashop shows the cart empty. And if I add a product using the normal "Add cart" button in the product list page,prestashop creates a new cart (i.e. the cart with id 35) and the cart I created (i.e. the cart with id 34) remains unused.

Edited October 5, 2017 by files___ (see edit history)