Get Cookies or Current User ID outside WP

Hello,

I’m trying to build a payment system with WP and I’m getting troubles to get a cookie value (or user_id) outside WP.

Here’s how the system works:

1- The user log in
2- When he click on “Pay” button a cookie is set with the product’s (post) ID
3- The user is redirected to the payment gateway to insert all his information.
3- Once he pays the gateway makes a call to an URL (/wp-content/themes/mytheme/success.php or /wp-content/themes/mytheme/cancelled.php) sending by POST all the transaction information
4- The user is redirected to the main website

The problem is that I’m not able to get the “product’s ID Cookie” from the success.php or cancelled.php. I also try to get the current_user ID but the result is the same.

I tryied using:

$user_id = wp_get_current_user()->ID;
$user_id = $_COOKIE['producte_id'];

Any idea?

Thanks in advance