Eric,
Not sure if I am following you correctly, but I did the following:
1. removed the code from the bottom of my checkout_success page and replaced with <?php echo $_SESSION['id_users']; ?>
2. Created a second page called "new.php and pasted all the code that was originally on the bottom of the checkout_success page into the bottom of new.php. And then followed it with <?php echo var_dump($_SESSION); ?>
Results were as follows:
When I got to checkout success:
2126
When I navigated away from success to new.php:
array(1) { ["id_users"]=> string(4) "2126" }
Still logged out after this. Issue remains. Any further help is appreciated.