Building a PHP script to create new image

I'm on my way to create a new script for my personal website. I want some kind of function where visitors can write something in an input field and then press generate image.

The script should then take a background image and insert the visitors text on top.

My question is: What is the php code to actually generate the image file and how do i connect the input data to the image?

I do not need full code examples if you don't have the time, i just need the basis practice of how to setup such a system.