It's now PHP 4.3.5 (I forgot to update it when it was updated for Drupal) that's the minimum requirement for Drupal/Ubercart. While PHP 4 is the minimum requirement, PHP 5 is what's recommended for Ubercart. Our testing and production servers are in a PHP 5 environment, so any problems that come up in PHP 4 won't show up immediately to us. In this case, Workflow-NG appears to be incompatible with PHP 4.3.9 (you might want to tell the maintainer of Workflow-NG about this), upgrading to PHP 5 should resolve this problem.