HipHop executes the source code in a semantically equivalent manner and sacrifices some rarely used features — such as eval() — in exchange for improved performance. HipHop includes a code transformer, a reimplementation of PHP's runtime system, and a rewrite of many common PHP Extensions to take advantage of these performance optimizations.
PHP has a number of key benefits that a lot of "advanced" and "properly designed" languages lack: a huge user base, existing open-source application code base, and ease of use that a web development language really needs. These things will keep people attracted to PHP regardless of how uncool or amateurish some pundits label it. So far, the main downside has been performance, partly addressed by APC, eAccelerator, Zend and C++ extensions, but this code transformation approach might now be able to give it the best of both worlds.
Having already gone mainstream production on the Facebook servers, we just need to see now how well it does on the entire PHP language and libraries to be truly transparent to PHP programmers.
