Right now aside from up and comers like Ruby, PHP is the top language for building dynamic sites or integrating custom apps into an existing site framework, and tying it into a server DB like MySQL. There are few things that you can't do with PHP. Knowing Java is great too, but Java is more on the multimedia/interactive end. Same with Flash coding in ActionScript.

It depends on where your interests lie, in the multimedia end or the site development end. Something to consider though is that if you learn multimedia scripting, you'll still need to know PHP and MySQL (r whatever RDB you're working with) to tie it all together in the site.