We updated the version of Guzzle used by the Marketing and Transactional PHP SDK to 7.2. This version of Guzzle requires PHP 7, so users of SDK versions after January 25, 2021, will need to use PHP 7.

We want to stay up to date on the libraries we use, even though that may mean dropping some legacy support. We came to this decision with lots of input from users on our Github projects; thank you for the contributions and feedback!