I also disagree strongly with that piece of advice. Even though, as others have pointed out, if you become a PHP developer you're going to have to deal with nasty legacy code; there's nasty code written in Ruby, Java, Python, etc. too. It's just nasty in a different way.

PHP is actually the language I do recommend people learn if they're interested in web development (obviously along with JavaScript), because PHP developers are in high demand. PHP developers can always find work, precisely because there is so much PHP out there.