I think it's great that you want to learn something new. The main question that I would ask myself is, what adds the most value to my portfolio?

How much value does another similar backend language give?

My thoughts on this would be: Not to much. I would rather look for something that makes you a more interesting engineer. You could e.g. learn to make android apps. That would make you a quite valuable person being able to code mobile frontend and a backend service. Or a backend language/framework that is far different and let's you solve problems that you run into with PHP on high traffic websites. Some companies started using Scala for high performance parts of their service. Or get a good understanding of Apache Lucene a search engine.

But something that is more different from PHP and will make you a more interesting engineer.