[ze_dude]

However, Facebook uses HipHop (http://developers.facebook.com/blog/post/358) to make PHP faster (so it's not the plain-vanilla PHP).

From the linked article:

HipHop programmatically transforms your PHP source code into highly optimized C++ and then uses g++ to compile it. HipHop executes the source code in a semantically equivalent manner and sacrifices some rarely used features such as eval() in exchange for improved performance. HipHop includes a code transformer, a reimplementation of PHP's runtime system, and a rewrite of many common PHP Extensions to take advantage of these performance optimizations.

[ultrasaurus]

Although it's safe to assume you won't have scalability problems right out of the gate either.

Once you have tens of thousands of hits a week presumably you can port to HipHop yourself ( http://github.com/facebook/hiphop-php ). Needing to scale is a great problem to have.

[robertross]

I completely agree, the best problem in the web world. That's why I'm trying to think of it now because I open the doors to a project I'm developing for myself.

What about the projects that use PHP extensions in the .ini to speed things up (such as http://pecl.php.net/package/APC)