I know a number of people who make a full time living either working as a programmer for a big company, or being self employed as a programmer and freelancing. One of them only uses PHP, and earns a fairly good salary for it as well.

It is hard to get into programming, and having a wide range of languages that you are familiar with will certainly make it much easier. The more things you can do, the more jobs there are you could do Java seems to be a logical step with current PHP programmers, and Zend seem to be pushing PHP and Java as well lately.

While many larger companies ignore PHP, lots of small businesses seem to be becomming ever increasingly aware that there are lots of php programmers out there, and who'll work cheaper than ASP/ASP.net guys - all in all, makes perfect sense. You can achieve almost anything in PHP these days, and so there's no reason for SB's not to choose PHP, and more and more seem to be.