Alvaro Videla has written up a response to Wade Arnold's Scala's easier than PHP post and some of the points he doesn't agree with him on.

Before going on with the points, let me state something: please avoid flame wars, all the Scala vs. PHP stuff, fanboyism and what not. This post is not about that. Regarding Wade Arnold I have to say that I fully respect him. While I don't know him personally, I know him for his work on AMFPHP, since it was a platform I used to work with before.

Alvaro talks about functional programming and some of the main points he had made about Erlang in a talk he had given - code reload, being ready for multi-core and no shared state between scripts. He talks about how these (really) apply to PHP.