So, why has Facebook rewritten the PHP runtime? Because PHP is obviously too slow for their tastes. A few years ago, I had a coffee meeting with some of the folks from Zend. When they asked what I had been hearing about PHP in the market from my sources, I hemmed and hawed, then told them that I had heard people complaining about how slow PHP was. Now, I don't personally consider PHP slow: it is simply not a language designed for the sorts of workloads that Java and .NET are.

But that still doesn't change the fact that PHP can be a tad pokey on the server. Well, when I said this to the Zend folks, their immediate reaction was similar to that of a gestapo officer looking for a spy: "What? Who said that? Tell us their name!"