As otherwise noted, you can also run your backend on Javascript, with the NodeJS server. That being said, I do recommend knowing more than one programming language - by having more tools in your toolbox you both make yourself more attractive to employers and by being familiar with more "coding philosophies" you'd be able to create better applications.

Javascript server programming is easy to learn - if you are already well familiar with Javascript; Typescript is a richer language that is based on Javascript and you'd often find it intermingled with Javascript in the same application, and it also runs with the same tools (such as NodeJS); PHP is a relatively simple language with a very large and friendly community, and tons of libraries; Java is also a very simple language with a lot of libraries, but is somewhat bit less accessible than PHP for "just building a web application backend"; Python is very popular these days (competes with Javascript for most popular programming language).