What is HipHop
HipHop for PHP transforms PHP source code into highly optimized C++. It was developed by Facebook and was released as open source in early 2010.
HipHop transforms your PHP source code into highly optimized C++ and then compiles it with g++ to build binary files. You keep coding in simpler PHP, then HipHop executes your source code in a semantically equivalent manner and sacrifices some rarely used features – such as eval() – in exchange for improved performance.
Facebook sees about a 50% reduction in CPU usage when serving equal amounts of Web traffic when compared to Apache and PHP. Facebook’s API tier can serve twice the traffic using 30% less CPU.
One of the explicit design goals leading into HipHop was the ability to continue writing complex logic directly within PHP. Companies with large PHP codebases will generally rewrite their complex functionality directly as PHP extensions in either C or C++. Doing so ends up reducing the number of people who are able to work on the company’s entire codebase. By keeping this logic in PHP, Facebook is able to move fast and maintain a high number of engineers who are able to work across the entire codebase.
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Reference : http://developers.facebook.com/blog/post/358/