Still no documentation and not anywhere close to done, but my #ActivityPub #PHP library is open source at https://github.com/pterotype-project/activitypub-php.
My goal is to provide a well-tested, flexible library that handles the complicated parts of ActivityPub (JSON-LD, persistence, addressing, etc.) in a way that can be plugged into an existing PHP codebase with no hassle. That'll help me refactor #Pterotype, and will also make it easy for other developers to make developers to make plugins for Joomla, Drupal, etc.