Thank you for reaching out to Constant Contact's API Support.
The redirect URI needs to match in both your Mashery account and what you write in your application for the redirect_uri parameter. As long as these match then you should not receive any errors for this specific parameter.
Thankyou for the reply. Is this the same case in client and server flow? In both the redirect uri of mashery account and app should match? If this is so then how can I handle multiple ctct accounts with a single api key and secret? I want to authorize users ctct accounts using my ctct mashery account app. Is this possible?
Yes it is possible to use the Client Flow for multiple accounts. This is actually what we use for our iOS/Android app. There is a link at the bottom of the Client Flow page to an example of how to set this up. For easier reference here is that link. https://github.com/shannon7wallace/OAuth-2-PHP-Example