Hello,   I had to take some time to look into this library as it is built for an older and now deprecated verison of our API. After investigation, I found that the way that UpdateContact is implemented, it will only work if the Contact object is created by using the  searchContactsByEmail method. If you do not get the contact details this way, then a required property of the contact object is not set, causing the UpdateContact method to fail.   If you are working on a new API integration, I would strongly advise working on our V2 API and using our PHP SDK that can be found here: https://github.com/constantcontact/php-sdk   If you have concerns about support for older versions of PHP, you can make use of some of the older versions of the PHP SDK that will have better support for older versions of PHP.   Sincerely, ... View more