Serializable objects disappearing from session, but not on localhost - php

Question

I have a shopping cart object that implements serializable, and it works fine on my localhost, but when I upload it to the live server it stops working correctly.
When I add a product to the shopping cart and print_r($_SESSION) I can see the products have been added successfully, but when I refresh the page they disappear. This only happens with objects that implement serializable. These same objects work perfectly on my localhost, which has IDENTICAL code, and it's driving me insane.
Could it be a difference in session handling between php version 5.5 and 5.6?
I am using spl_autoload_register to load my classes if that helps at all (none of them appear in the session as incomplete_class or anything like that).
I also noticed that the session save path is blank on the live version, but is set on my localhost - but the rest of the session functions perfectly - it's only when it contains serializable classes that everything disappears.
Please help before I murder someone...
OK here's the code:
shopping cart class:
class shoppingCart implements Serializable
{
private $products, $error;
function __construct()
{
}
function addProduct($productCode, $quantity = 1)
{
include_once ('include/requests.php');
$productInfo = requestPostData('getProductList');
if (in_array(strtoupper($productCode), $productInfo['results']) == true)
{
if (isset($this->products[strtoupper($productCode)]))
{
$this->products[strtoupper($productCode)] = $this->products[strtoupper($productCode)] + $quantity;
return true;
}
else
{
$this->products[strtoupper($productCode)] = $quantity;
return true;
}
}
else
{
$this->error = 'Product '.$productCode.' could not be found.';
}
}
function editProduct($productCode, $quantity)
{
if (isset($this->products[strtoupper($productCode)]))
{
$this->products[strtoupper($productCode)] = $quantity;
}
}
function removeProduct($productCode)
{
if (isset($this->products[strtoupper($productCode)]))
{
unset($this->products[strtoupper($productCode)]);
}
}
public function getProducts()
{
if (count($this->products) >= 1)
{
return $this->products;
}
else
{
return false;
}
}
public function isInCart($productCode)
{
if (isset($this->products[strtoupper($productCode)]))
{
return true;
}
else
{
return false;
}
}
public function getQuantity($productCode)
{
if (isset($this->products[strtoupper($productCode)]))
{
return $this->products[strtoupper($productCode)];
}
else
{
return 0;
}
}
public function getError()
{
$error = $this->error;
return $error;
}
public function resetError()
{
$this->error = '';
}
public function serialize()
{
$dataArray = array('products' => $this->products, 'error' => $this->error);
return serialize($dataArray);
}
public function unserialize($data)
{
$dataArray = unserialize($data);
$this->products = $dataArray['products'];
$this->error = $dataArray['error'];
}
}
And the code on the page:
if (session_status() !== PHP_SESSION_ACTIVE)
{
session_start();
}
if (!isset($_SESSION['shoppingCart']))
{
$_SESSION['shoppingCart'] = new shoppingCart;
}
if (isset($_POST['cart__add_to_cart']))
{
if (isset($_POST['cart__quantity']))
{
if ($_SESSION['shoppingCart']->addProduct($_POST['cart__add_to_cart'], $_POST['cart__quantity']))
{
header('Location: http://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']);
}
}
else
{
if ($_SESSION['shoppingCart']->addProduct($_POST['cart__add_to_cart']))
{
header('Location: http://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI']);
}
}
}
If I print_r($_SESSION) right at the bottom of the page after a product has been added then it appears in the session as it should... But as soon as I refresh it disappears :'(