What are possible causes of the error message, "expired authorization code," when requesting an OAuth Access Token?

I'm testing the cookbook code at, "Interact with the Force.com REST API from PHP."
http://developer.force.com/cookbook/recipe/interact-with-the-forcecom-rest-api-from-php

At oauth_callback.php, line 31, I seeing:

• status: 400
• error_uri: expired authorization code
• error: invalid_grant
• error_description: expired authorization code

Thanks.