SQL isn't a language by itself and syntax and structure can vary wildly.
Java, C#, and Python web jobs easily outnumber PHP jobs.
Python, or Rust are probably used for very few desktop applications, compared to their system or web use.
You still need to choose a UI framework if you decide to build a desktop app in any one of those, except C#.
The desktop UI framework you choose might even use HTML/CSS anyway.
You probably still need to choose a framework in Java or Python for the same reasons you would on the frontend. (Maybe Rust too?) Code structure, consistency, supportability, and no wheel invention.
If you chose PHP for your backend and the project started after 2008 or so, your choices are bad and you should feel bad.