I'm a seasoned PHP programmer and I really like the way it works and find it very fun to work with (performance could be improved and some functions renamed, but nothing too serious).
However, I took a java seminar and now Im very interested in using GWT for upcomming projects, although I think the learning curve can be steep.
Should I really go through with this change (PHP > JAVA)? Where to begin?
Why not try Ruby or Python? That's what I did, and I love every bit of it. Using django for python on a few sites.
I don't see why Java would be such a natural way to go, just because it got a huge userbase
perl is a fair bet, too. it looks a bit like php, too, which can be a plus or a minus depending on how you look at it.
Why should you go from PHP? Instead of going to another language try to learn new techniques with the language you already know.
Try and install some framework (I recommend Codeigniter), try to code in OOP and MVC style (this is also the only way when working in framework :)
But if you want to change language, then go to Ruby - it's a beautiful language and Ruby on Rails is a beautiful framework. :)
I would second considering going even deeper into php. MVC Frameworks (CodeIgniter, Kohana, Cake PHP, writing your own), proper OOP practices especially abstracting and code reuse that enable scaling big, template engines like smarty, learning to profile and optimize PHP code, managing PHP code with git or other DVCS's. Work with other talented developers, contribute to open source PHP projects, get involved in a framework by testing other peoples plugins or writing your own for the framework you actively use. If you're really talented, contribute to the PHP project itself. Change your PHP developer day job every 2-3 years early to mid career to work on many varied projects with other talent.
You haven't provided much info about how you work with PHP and on what size of teams/projects, but I will say if you've never worked with other highly talented developers and share solutions to problems you are all working on together, where you can all expand each other's knowledge at an accelerated rate, or developed mission critical applications that have to scale to tens or hundreds of thousands of users - like if all you do is small business sites for example - then there is a lot more to learn and do, and have fun doing.