Signature invalid

I keep running into this OAuth error.

I started out using the Zend_Consumer_Oauth library, but thinking it may be a problem with that, I've tried the library at http://oauth.net/, the official PHP SDK, and the scripts at http://nullinfo.wordpress.com/oauth-yahoo/ which I've seen referred to on this forum a few times.

All get the same error: Error requesting oauth request token, code 401, message: oauth_problem=signature_invalid

Can anyone get any of the above libraries working at the moment?