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Hack is a language for HHVM that interopates seamlessly with PHP. HHVM supports both Hack and PHP; code can be run on HHVM in either language or even a mixture of both.
To get started with Hack, it really is as simple as:
Use <?hh at the top of your file; you can also change <?php to <?hh in existing PHP files, and your code will run just as before in HHVM.
Optionally name your file with the .hh extension to distingusih your Hack files from your PHP files. Of course, you can keep the name of the file .php (or any other extension that you use)
Optionally start using Hack features