$PHP_SELF issues

It's better form to pass dependencies in.  It also makes it much easier to test.  `global` can be used as a workaround during the transition:
```php
function zen_get_shipping_enabled($shipping_module, $order = null, $command = null) {
  if (!is_object($order)) {
    global $order;
  }
  if (empty($command)) {
    global $zcRequest;
    $command = $zcRequest->readGet('cmd');
  }
  // ...
```