The simple answer to your question is that of supply and demand. Basic economics really.
There are thousands of projects done in PHP and because it's low learning curve entry rates are pretty high. Because there are so many PHP developers, the salary drops. There are fewer Java developers I am willing to bet so that increases their salary, in theory.
Again, there are literally 1000's of PHP projects, both open source and closed so I don't think the industry is going to ignore PHP as a technology. As long as the guys at Zend keep PHP up to date and develoeprs keep using it there will be a market. Basically it's up to you whether PHP lives or dies. When programmers top using it and begin porting applications to other platforms, then start to worry.
PHP ripped the carpet out from underneath Perl and I think it's gonna stick around for a while -- certainly it won't be de-thrown(sp?) by Java. It'll be some up start like a newer Ruby with some kick ass framework and IDE that steals it's top spot. The fact of the matter is, PHP does what it does really well. It attracts some interesting developers and lets millions more feel like programmers.
IMHO there are simply to many reasons for PHP to stay and not very good arguments for it to disappear. PHP projects tend to be smaller than Java applications, so even as lead developer you won't make a killer living as a programmer. Start your own business if you want big money.
Programming is a passion for most. It'll be hard to compete in a world where the next guy will do something for free out of desire, so if your after money, I suggest leaving software development and taking up banking or something.