You're doing yourself a massive disservice to know PHP and only PHP. There are literally hundreds of other languages out there, some good, some bad, but all of them have something to teach you.
Remember a programmer is not necessarily interested in programming, but often in having programmed, where the end result is paramount.
When I see "PHP programmer" it's all too easy to jump to the conclusion of someone who's too timid to go outside their little bubble and see how other people get things done.
I'm definitely interested in other languages but as long as I feel I can do the same with PHP it's just wasting opportunity costs to spend much time on it (I could've build a whole new startup in that time). The day PHP doesn't cut it for me anymore is the day when I need to use another language. And by then, I definitely will.
Whilst there is definitely an advantage to using tools that you already understand,
you might be surprised how much of your current skills are transferable to a different language.
If nothing else , learning another language will help you write PHP because PHP uses a number of paradigms , all of which are done better by other languages. So learning another language that is more tightly coupled to (say) Object Orientation (Ruby/Java) will help you understand program design better.
Depending on the size of your codebase , I think it's possible to overestimate the cost of changing tools.
For example, I have a ~100Kloc PHP codebase. When it was around ~20Kloc we had considered re-writing it in a different language. We decided it wouldn't be worth the time spent, I now kick myself for making that decision :)
Would you go to a hospital where all the equipment was at least thirty years old, where the newer medications weren't available because the old ones were good enough in the opinion of the head doctor? Learning about the new equipment, techniques and medication is just wasting time that could be spent on treating patients, right?
Whenever I patrol the PHP section of Stack Overflow it's like teleporting back in time a hundred thousand years and watching people bang rocks together in the hopes of making fire.
People are trying to do things that should be simple but are needlessly hard because of PHP's language design and the poisonously bad ecosystem. It's because most programmers in the PHP universe always build directly on top of the primitive library functions. For some reason this is the standard way of doing things.
The things you can get done in a framework like Django or Rails with a few off-the-shelf add-ons and some duct tape is so impressive it's somewhat terrifying. To do the same in PHP would take weeks, and even then, only if you know what you're doing.
Remember a programmer is not necessarily interested in programming, but often in having programmed, where the end result is paramount.
When I see "PHP programmer" it's all too easy to jump to the conclusion of someone who's too timid to go outside their little bubble and see how other people get things done.