Work in both...
If you can do Java/C# you can pick up PHP 5,Actionscript 3 pretty quickly. If you do PHP and want to learn cross skills then concentrate on 5 and learn interfaces/abtsraction etc. Learning cross skills is the important thing as technology is faddish but the underlying methods are not so(how many languages now use the same/similar object syntax?).
Though it can be quite hard as once you are pigeon holed in one thing it can be hard to escape. It also depends on the quality of mentoring available to you. Good PHP mentoring is better than bad Java mentoring.
