It’s not news that Facebook has been running a modified version of PHP for some time. What is news is that now so can you.
You can get a copy of it at this website:Hack. It installs on most OSs (or will, soon), and even runs in the Cloud (on Heroku).
According to this, Facebook Introduces ‘Hack,’ the Programming Language of the Future | Wired Enterprise | Wired.com, what you get is a high performing version of PHP with very little you have to change to get the performance gains.
It will be interesting to see if this helps to drive up usage in PHP. There are lots of new technologies to build web site with these days: this could make PHP a desirable option.