Depends on why you're a programmer. If you wish to optimize for compensation, by all means, learn as many different languages, platforms, and technologies as you can.

So I would say you should learn something in addition to PHP so that you bring more value where you go.

Put another way: don't be a PHP programmer. Be a programmer. Then, use the right tool for the job based on the constraints given to you. If that ends up being PHP, you'll still be paid more to do it.