Hack is a language for HHVM (HipHop Virtual Machine) that interoperates seamlessly with PHP. The barrier to entry for Hack is low.
The goal of Hack to offer developers a way to write cleaner, safer and refactorable code while trying to maintain a level of compatibility with current PHP codebases. The primary way that this goal is achieved is to provide developers a way to annotate PHP functions and classes with type information, providing a type checking tool to validate those annotations. In addition, Hack also provides specific language feature additions such as: Type Annotations, Hack Modes, Generics, Nullable, Collections, Type Aliasing, Async, Continuations, Traits, Lambda Expressions and Tuples