Save dynamic php image

I've been working off some code I found and have made it work for my needs.

What it does is overlays text over an image using DTR

I've setup a form that uses the GET method to change the text color, text, font etc.

so the image src looks like <img src="preview.php?text=SampleTextcolor=#FF0000">

I am looking to figure out how to save that image and assign it a auto-increment ID number

But being that the image is dynamic, I can not figure out how to do so. Any help on this matter would be a appreciated.