Great and interesting article. Nice job!
Contributing to your article, I agree totally with you. I think are not only one way to be a better developer, are so many ways to reach that goal.
If you like PHP and you want to be better at that programming language, focus on that language, learn everything about that (i.e. frameworks, design patterns, better practices, more, more and more).
If you like Machine Learning, do exactly the same thing.
What I think it makes some dev a better developer or a great developer or a ninja developer. . . is the capability to solve problems. Think in this, everything that a developer does is related to solve problems for the people and even for the same devs. That ability to solve complex problems and create effective solutions with order, with good practices and making the things easy for his users with no matter what language or framework, is what makes us better developers. Because at the end, we are all developers solving problems and is the only thing that matter.
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