Which language would annoy a PHP user?

We have nobody who coordinates the programmers at the company. Everybody uses the technology he wants.

Which programming language would you suggest to learn that would annoy a PHP user (who knows a bit of Java) and give him a bad time if he has to take over a project?

Everything constructed by real computer scientists.
Take Haskell + Yesod. When asked why by your boss, you can tell him it's because of superior security and scaling than pretty much everything else.