You need a sound understanding of CSS, HTML and javascript. Although you may have been messing about with them a lot, try and get an in depth understanding of CSS selectors, what semantic HTML mark up is, and how to write quality code in javascript -- unobtrusive javascript is something to look up.

Javascript is really worth learning in depth. It has some quirks that will gently introduce to some useful topics such as JSON, closures, elements of functional programming, prototype based inheritance, and so on. Further, it will help you understand how to get more out of other dynamic languages too; python, ruby, etc. Douglas Crockford has some great videos on Javascript that are worth looking up.

You're not restricted to client-side programming with Javascript either. For instance, look up nodejs - a nascent server-side javascript framework that looks quite promising. It will really make you think if you've just been a PHP guy up to now!

Try and get a handle on PHP's more advanced aspects - many of which have only really arrived in recent versions. Classes, name spaces, closures and so on. Although I'd really suggest another dynamic language altogether; python is a good choice, but ruby is splendid too.

Once you're confident with that stuff you may want to go in a slightly different direction. Try Java, C, Clojure or Erlang. Try reading some more general computer science books.

I hope that helps. Good luck and all the best.