The SitePoint PHP blog has a new post today that asks can we expect HACK and HHVM to replace PHP as an evolution of the PHP language and interpreter.
HHVM is intended to achieve both parity with the Zend Engine features and best possible performances. Facebook claims a 3x to 10x speed boost and 1/2 memory footprint by switching from PHP+APC to HHVM. Of course this is really application dependent (10x being for the FB code base). [...] Instead this article will focus on HACK which is an evolution of the PHP language designed to be safer, to enable better performance and to improve developer efficiency.
He starts off by helping you get an instance of HHVM up and running (via Vagrant) and create a simple HACK script. From there he gets into some of the more advanced HACK features like constructor argument promotion and collections. The talks some about typing, type hinting and the use of generics as well.