Request: http://api.search.yahoo.com/WebSearchService/V1/webSearch?appid=dclifton&query=php+xhtml&output=php
Results:

Array
(
    [Error] => Array
        (
            [Title] => The following errors were detected:
            [Message] => Array
                (
                    [0] => The service has been shut down. For further details, please see the Deprecated Services blog post http://developer.yahoo.com/blogs/ydn/posts/2010/08/api_updates_and_changes
                )

        )

)