The Facebook www code is written in Hack, which started off as a PHP-compatible language / stdlib, and then dropped PHP compatibility when it was holding them b

The Facebook www code is written in Hack, which started off as a PHP-compatible language / stdlib, and then dropped PHP compatibility when it was holding them back. Having worked with both, any one of the major additions (XHP, good type annotations, sane collection types, etc) makes my development experience 10x better; going back to vanilla PHP now just makes me sad...