Been a while since I last heard about facebook’s HipHop PHP optimizer project. First time I have heard of it it was a compiler from PHP to C, something I have already ran into with another interpreted language – TCL/TK, and is mainly beneficial for projects that once the interpreted code (Iie PHP code) is stable and shipped there is no need to modify it. In other words you lose the ability to modify your code on a whim that is the reason why most sites today use interpreted languages.
I was actually surprised to learn that the main reason facebook was unhappy with the compiler is that the deployment of a compiled code was resource intensive and since facebook is pushing a new update once a day they started to look into other alternatives to compiling their code into machine code.
But if it is faster, how come it will not make your site faster? To understand this you need to keep in mind how facebook’s scale and how it works works.
Facebook had at some point 180k servers A 1% optimization will allow them to save 1800 servers and the cost of their electricity and maintenance. My estimate based on pricing by web hosting companies is that this might amount to saving 100k$ each month. So facebook is more likely doing this optimization to reduce cost and not to improve side speed, but for lesser sites a %1 optimization will not be enough to avoid the need of upgrading your hosting plan and even if there was a cost benefit it is unlikely that for most sites the savings will be worth the amount of time that will need to be invested in changing to use HHMV and testing your site on it, especially since it is not a fully mature product yet (just because it works for facebook doesn’t mean it works everywhere)
The other thing to take into account is that by its nature facebook can do a very limited caching as essentially all the visitors are logged in users. They can still keep information in memory in a similar way to how the object caching in wordpress works, but they still need a PHP logic to bring it all together, while wordpress sites can use full page caching plugins like the W3TC plugin which produce HTML pages that serving them bypasses entirely the need to interpret the PHP code and therefor improvements in PHP interpreting is of very little importance to those sites.
It is not that HHMV is totally useless outside of facebook, just that its impact will be much bigger on bigger and more complex sites then most wordpress sites tend to be. The nice thing about it is that it is open source and therefor the can adopt the PHP JIT techniques from HHVM into the core PHP interpreter.