[lacker]

Well, at some point Facebook had a code base with a whole bunch of PHP and it was worth a lot of effort to make it more efficient. One possibility was rewriting stuff, one possibility was this project to make a different PHP compiler. It wasn't obvious what the right strategy was and the Facebook spirit is to try lots of stuff and see what works. So effort went into each, and basically each of the methods turned out worthwhile for some things. Nowadays a lot of components of Facebook are not written in PHP, especially the infrastructural components. But a lot are written in PHP and work goes into the compiler. So the answer is, it didn't happen dogmatically, it happened bit by bit.