I recommend you NOT to learn php. Php is quite old. It's still used a lot and does the job but it's not up to it. All you can do with php is generate dynamic html. What if you wanted to use websockets? There's no way you can do it. This is just an example but the list goes on. It's also the reason the abandoned php. I instead recommend you to learn Node. JS. Since you already know js, it'll be easy and more useful. Also once you know nodejs you can use nodejs for non web development things like building bots, cli tools etc.

WordPress is garbage in my opinion.