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This is the best advice on career management. Being able to break out of being "a php engineer" or "a java engineer" lets you target jobs and companies much more freely and ultimately lets you figure out what interests you as a developer in more real terms (and lets you go after it). It also sets you up to lead efforts on solving problems, rather than defining solutions in a more narrow window
My experience is the opposite, interestingly. I used to know and use a wide range of languages. Being able to say "6 years of professional Scala" has opened more doors, and focusing on a single language has allowed me to learn techniques and algorithms rather than worry about the differences between languages so much.
A colleague that I really respect told me at my last job that his career was a series of broadening and deepenings. He started with a deep knowledge of Java, but found that success came as he branched out into Python, Ruby, and Scala. At that point, he discovered that developing a focus on distributed computing with Scala was the key to the next level, and so on. I think half the challenge is identifying when its time to do one of those two.