Back in the day I had plenty of stuck-in-their-ways Java and J2EE developers tell me that PHP isn't scalable. They were wrong, and I politely told them that. It was a classic case of people reciting opinions without really having explored the truth. I never heard anything like it from any developer that had actually used PHP for developing a project.
PHP's interpreter is written in very fast C; it has an extremely low memory footprint, and its execution leaves no traces behind. What's there that won't scale? Slap an opcode cache on the server, parameterize your SQL queries, and it gets even better.
Today, thankfully, I don't hear much of that nonsense anymore, but I felt compelled to reiterate the point today when I was reminded that Wikipedia is built on PHP and MySQL, both major open-source projects. As of this writing, Wikipedia is the 7th most popular website in the world according to Alexa. I believe a great deal of Facebook is also PHP, if not all of it.
PHP is definitely not the answer to everything, and as an architect I generally rather prefer the rigidity of a Java or a C# when creating a system. But to say that PHP is not scalable is absolute nonsense.