Facebook uses PHP for the user-facing portion of the site. Here's a link where they discuss some of what they do
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.
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.
What about the projects that use PHP extensions in the .ini to speed things up (such as http://pecl.php.net/package/APC)