[API WebService] get orders sorted by date

[zion 1]

Hello, I am developing a website to receive orders from several prestashop.
I bring the orders from php with;

$ opt = [
                     'resource' => 'orders',
                     'display' => '[id, reference, total_paid, id_customer, id_cart, id_address_delivery, current_state, payment, date_add]',
                     'sort' => '[id_DESC]',
                     'limit' => '10'
                     ];
             $ xml = $ webService-> get ($ opt);
             $ resources = $ xml-> orders-> children (); 

I have tried to change the sort to "date_add_DESC" but it does not return anything.
I have thought about ordering the array when I have filled it with php's usort () function,

            usort($ArrayPedidos, function ($a, $b) {
                return strcmp($a["fecha"], $b["fecha"]);
            });

but I can't, any help would be good for me. thanks.

Edited January 21 by anderrios (see edit history)