From PHPEverywhere:

Infoworld has a series of articles on IT myths. There is an interesting comparison of Java and PHP, using Friendster as a case study in IT Myth 6: IT doesn't scale.

As they say, scalability is a matter of brain matter and skillful cooking: "In the end, scalability isn't an inherent property of programming languages, application servers, or even databases. It arises from the artful combination of ingredients into an effective solution. There's no single recipe."

He talks specifically about PHP's stateless "shared nothing" approach to its architecture, and how that really effects the scalability of your applications. In the end, it's really just a combination of all of the pieces of the puzzle that makes scaling an issue...