Among the many sites I follow for programming discussion is /r/PHP on reddit. While most of the discussion is more user-based than I would like - things like frameworks, use of PHP-based software packages and the like are usually discussed more often than actual programming - there are occasionally a few gems worth chiming in on. But it never fails that, at least once a week, I see the headline "How do I become a PHP developer," or "What do I need to know to be a PHP developer?"
My answer is simple: don't. Just stop.
Don't be a "PHP Developer." Don't be a "Java Developer." Don't be a "Ruby Developer." In fact, don't be any kind of developer that depends solely on a single language.
Languages come and go. Ten years ago I would bet the majority of web programming was still done in Perl. Fifteen years ago the web was still widely misunderstood and Java was promising that we would only have to write code once to run on any computer. Twenty years ago you found C, FORTRAN and COBOL on mainframes.
Every few years a new language comes around and everybody moves to it. Sometimes they stay around, and sometimes they don't. C has been around for many years and is just as valid now as it was twenty years ago. Even if you're programming in C++ or Objective-C (both of whose roots go back further than you probably realize), you still need to understand the fundamentals of the C language. Will we still be using Clojure in 20 years? How about Coffeescript? Who knows. Maybe. Maybe not.
My point is, don't chain yourself to a single language. If you do that, you will be forever behind the curve.
A good developer should be able to work independent of his/her tools, should be always willing to learn new and exciting things, and should be able to apply lessons learned in past development independent of the language they are working in. A good developer should be able to come up to speed quickly on a new language. And while it is true that every developer will probably always have a preferred language and a language they're best at, we as developers should always place the craft of software development ahead of specialization in a single language, and we should be willing to use the best tool for the job independent of our linguistic preferences.