Nic Rosental is a developer based in Atlanta and he recently gave a great presentation on how to build a really fast server stack.
Nic defines a stack as, "The components that make up your application. From the web server to the code."
Nic starts with a LAMP stack (Linux, Apache, MySQL, PHP). He explains that the normal LAMP stack isn't optimized for speed and then he goes to look at a wide variety of server setups and tools.
During the presentation he compares PHP 5.3 to PHP 5.5 for speed. If you use PHP at all, you really should watch and see how much faster 5.5 is." frameborder="0" allowfullscreen>