Seeing as you want to grow as a programmer then I would encourage you to first excel in PHP. That means spending time looking at in the in-depth ways of doing things, but more importantly the best way to do it. Look at best practices for PHP, how to improve your coding style and to work on learning how best to refactor and improve older code within systems to be performant and secure.

I would also encourage you to look into the pros and cons of PHP, as there are situations where some other language is better. Sure you can do it in PHP anyway most of the time, but you should always look for the best solution, so using PHP may cost you more time or risk security or some aspect for a specific task. I don't know what you do and PHP could be perfect for everything you do, but knowing limitations is a good thing to know anyway. Once your happy with your level of knowledge in PHP that's when I would start looking at other languages, as you never know what you'll like. I've always been fond of java/.net stuff, but I recently tried Node and I love it, so you may find other things that you enjoy coding in.