PHP is another language altogether, unrelated to Java. There are actually many other alternatives besides these two: Microsoft's "Active Server Pages" (ASP), erb (embedded Ruby), and many, many more. It's certainly useful to learn about multiple alternatives, but it's probably best to wait until you've gotten some facility and understanding of one.
I've moved to Java for a series of reasons some of which have been mentioned Gregg. There is one I'd like to add though. Code reusability on the desktop and mobile devices. After working years with two different code bases one in Delphi and one in php we realized we were not getting anywhere any time soon. Java allows for the same codebase with different presentation layers. Saves a lot of time developing. Add to that tools like ORM and XML and things fly.