shopping_cart.php error when changing quantity

[lismith]

I searched and didn't find anything about this problem. I'm getting this error when quantity is changed in the shopping cart:

Warning: Illegal offset type in (path)/includes/classes/shopping_cart.php on line 495

Warning: Cannot modify header information - headers already sent by (output started at (path)/includes/classes/shopping_cart.php:495) in (path)/includes/functions/functions_general.php on line 44

I haven't made any modifications to either of these files, and I reuploaded them thinking they may have been corrupted.

Where else could this error be coming from? I don't have a clue where to look.

Line 495 in shopping_cart.php is the 3rd line in this block of code:

  function get_quantity($products_id) {
    $this->notify('NOTIFIER_CART_GET_QUANTITY_START');
    if (isset($this->contents[$products_id])) {
    $this->notify('NOTIFIER_CART_GET_QUANTITY_END_QTY');
      return $this->contents[$products_id]['qty'];
    } else {
    $this->notify('NOTIFIER_CART_GET_QUANTITY_END_FALSE');
      return 0;
    }
  }

and line 44 of functions_general.php is:

    header('Location: ' . $url);

HELP!?