Any claim that any language that has basic control structures and access to OS facilities can't be made to scale is absurd on its face. When you say that a system scales well, you're talking about its asymptotic behavior. Using an inefficient language implementation will penalize you by at worst a constant multiple.
I have never understood peoples thoughts about which languages scale better than others, because all web languages do the same thing - process data and return it to the user.<p>The only thing you need to worry about is caching and sessions, but that's not a language specific task.<p>To scale any web language, you just need to add more servers to the cluster. Its the database which is the real scaling problem.
PHP's scalability isn't the big question. It's not the thing that gives people pause when they hear there's a PHP project looking for a developer. As far as dynamic languages go, PHP scales better than most and performs pretty damn well out of the box. But it's whether or not you can be productive in PHP that's the real question.