Here at my employer Phone.com, we are building a new Hypermedia API, and we've settled on Mason for the media type. Just yesterday we open-sourced a PHP library for generating Mason data structures (draft 2 format) suitable for feeding into json_encode() for rendering Mason response bodies. Check it out!
https://github.com/Phone-com/mason-php . It's already available through Composer too. "composer require phonecom/mason-php"
This is the first Mason tool that we're aware of within PHP land. Hopefully it will promote adoption within this community.
-Curtis Farnham
Senior API Developer
Phone.com