HHVM Zend Extension Source Compatibility Layer

Is anyone familiar with the: HHVM Zend Extension Source Compatibility Layer? https://github.com/facebook/hhvm/tree/master/hphp/runtime/ext%5zend%5compat

I read about it on this HHVM article.

[edit] Thanks, fixed it. Markdown doesn't seem to work for the link.

Correct link: https://github.com/facebook/hhvm/tree/master/hphp/runtime/ext%5zend%5compat

Apparently you must write a Hack (facebook's php with type hinting) file with all the classes and functions that the extension defines.