I'm going to buck the general trend here, and recommend that your next language not be Perl, Python, or Ruby. If you really want to become a well-balanced programmer, you should either learn a lower-level, compiled language like C or Java, or you should stretch a bit and try Lisp/Scheme or Erlang.

Working only within the "scripting language" space of the typical LAMP stack will limit your thinking in ways that could hurt you later on.