How to make php change the code depending on the device and screen size?

Question

HTML

How to do something in php like in css the code changes depending on the screen size or device?
For example, if your screen size is more than 800px, then one action was performed, and if it is smaller, then another.

Answer 1

a bunch of libraries for device detection, for example mobiledetect.net

Answer 2

Complete nonsense. The server should not know anything about the clients