This might not be the most popular language, but Hack was developed by Facebook as a way to extend PHP through the introduction of static typing, new type hints, and enhancing the use of type hints. In that sense, Hack is an implementation of PHP that lets developers use both dynamic and static typing through a system called gradual typing. 

Hack is a programming language for the HipHop Virtual Machine (HHVM), created by Facebook as a dialect of PHP. The language implementation is open-source, licensed under the MIT License.Hack allows programmers to use both dynamic typing and static typing. This kind of a type system is called gradual typing, which is also implemented in other programming languages such as ActionScript.