Yesterday marked a major point in the evolution of PHP and its ecosystem. Facebook released their version of PHP, Hack, based on the work they've been doing with the HHVM (HipHop Virtual Machine) and compiler.
Hack is a programming language for HHVM that interoperates seamlessly with PHP. Hack reconciles the fast development cycle of PHP with the discipline provided by static typing, while adding many features commonly found in other modern programming languages. Hack provides instantaneous type checking via a local server that watches the filesystem. It typically runs in less than 200 milliseconds, making it easy to integrate into your development workflow without introducing a noticeable delay.
One of the key features is that it mixes well with PHP and will feel very familiar for those already used to using PHP. The homepage for the language includes all the details you'll need to get started with it, including an interactive tutorial walking you through some of the basics. Some of the features included in the language are things like type annotations, generics, native collections and lambdas. You can find out more in their official announcement.