https://github.com/gentics/mesh
https://getmesh.io/docs/beta/administration-guide.html#_lice...
Contentful.com seems to be the leader.
Graphcms looks nice, GraphQL is a promising technology to only fetch the content/relations you need to render that page.
I was thinking about using Airtable for even a simpler MVP
If I'm going headless, the last thing I want to do is setup a PHP stack.
It's certainly no more work than setting up nodejs, ruby, python, a go compiler, etc.