For the “Python isn’t fast enough for production backend” crowd from the same company that brought you the largest social network built on PHP + MySQL.

You mean the company that had to create their two PHP implementations and a translator to C++, to actually scale?

Facebook had hundreds of millions daily active users before writing a compiler.

> Facebook had hundreds of millions daily active users before writing a compiler.

In other words, Facebook felt compelled to write a compiler when they had hundreds of millions daily active users.

If you have hundreds of millions of daily active users then needing to write a compiler isn't the end of the world.

Compiling PHP into machine code sounds pretty much impossible, are they using some subset of the language? Does the compiled code use a garbage collector? Or reference counting?

They're using a derived language called Hack.

> Does the compiled code use a garbage collector? Or reference counting?

Not sure but it's open source so I'm sure you can dig up the answers one way or another: https://github.com/facebook/hhvm