I'm working on proper documentation for extensions as we speak, but for now you can find some very basic info at https://github.com/facebook/hhvm/wiki/Extension

I'm working on proper documentation for extensions as we speak, but for now you can find some very basic info at https://github.com/facebook/hhvm/wiki/Extension-API https://github.com/facebook/hhvm/wiki/Extension-API . As the author of pretty much THE book on writing PHP Extensions ( http://amazon.com/dp/067232704X http://amazon.com/dp/067232704X ), I can say with a fair bit of authority that writing HHVM extensions is SO SO SO much easier.