Remove:
<?php
if (!isset($_SESSION)) {
session_start();
}
?>
it isn't needed.
You can also remove:
<?php
if ($sosume->IsEmpty()) {
$sosume->redirStr = "store_cart.php";
$sosume->cartAction = "RedirectIfEmpty";
}
?>
although it is odd you are getting an error, since that should be defined in the same place as the rest of the cart.