In this recent post to Reddit, user JordanLeDoux shares some of the basics behind the Hack language (from Facebook) and making a "first jump" into it and its strong typing handling.
codebase. Having built HHVM, they wanted something that would enforce certain behaviors for developers that didn't rely on IDE's interpreting phpDoc statements. Hack's most interesting and largest function is that it adds optional strong typing to PHP, by examining the tokenized code and ensuring that where declared strong typing is respected.
Example code is included showing the different levels of typing and how to use them in a few example functions. He introduces some of the basic types included in Hack (like int, float and bool) and some of the types unique to Hack (like mixed, tuples, resource and closures). There's a brief look at maps, vectors and sets and a link to more documentation if your interest has been piqued to learn more and make that "first jump".