On the Erskine Labs blog today there's a new post from Wil Linssen with a suggestion to PHP developers who specialize in the language - take the time to learn another language and broaden your skills.
So many languages share fundamental principles, and indeed many are even derived from the same concepts, but because they differ in their approach there's something new to understand in each. It's these differences that will frequently bolster your understanding of your primary language, or indeed of programming as a whole.
He notes that it's not just PHP developers that pigeon hole themselves into one language - developers on other languages do to. He recommends they lift their heads up and branch out and stop attacking other languages with "language X is best" articles.
It's stepping outside of your comfort zone and engaging with the underlying concepts of the languages and tools you are using that will enable you to grow as a developer.