The IO Docs API console does not actually perform OAuth correctly and is unfortunately not a good model on how to do OAuth - we have a replacement coming very soon.
The source code for that PHP OAuth implementation looks good, so I would check the following things:
- client_id is your Mashery username
- client_secret is a 32 character long secret associated with the api key you are providing - you can view it here https://developer.surveymonkey.com/apps/mykeys
Edit: According to our logs, it looks like you've solved this problem, correct? I think you were providing the api_key as the client_secret.