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?

>>24
If you're talking about being able to do something like:

data Shape = Circle Float Float Float | Rectangle Float Float Float Float
Where Circle and Rectangle are your "tags" and Shape is your union, well, that does nothing to change the fact that GHC will do type erasure.