CloudKit Server-to-Server authentication

Question

Apple published a new method to authenticate against CloudKit, server-to-server. https://developer.apple.com/library/content/documentation/DataManagement/Conceptual/CloudKit

Answer 1

I made an working code example in PHP: https://gist.github.com/Mauricevb/87c144cec514c5ce73bd (based on @Jessedc's JavaScript example)

By the way, make sure you set the date time in UTC timezone. My code didn't work because of this.