

HHVM vs. Zend Engine in PHP 6 - BitAPIHub
http://www.sitepoint.com/hhvm-vs-zend-engine-php-6/

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preillyme
The ability to statically type in the most recent version of HHVM, via their
internally developed Hack “language”. Hack is a derivate of PHP, but supports
static typing. You start files you want to type check before compilation with
<?hh instead of <?php, and the compiler does the rest. This approach further
increases the quality of compilation, making native code that much more
efficient, and HHVM’s PHP that much faster.

