If you really want to excel and learn, pick up Java. It is so completely different from PHP that you'll learn a tonne (and likely appreciate PHP more).
The upside is that there is always a market for Java developers (not that there isn't one for PHP but it's good to have options).
I'm sure that the script kiddies will bury this post and tell you to learn Erlang, Ruby, etc (which are all good in their own right) but that's this old cat's 2 cents.