PHP, manipulating very large image files without loading them into RAM

I searched for this for a while, but I did not find what I wanted, so here is my problem:

using PHP, I want to create a very large image file, say 20,000 gigapixels, then I want to add a small image to a specific location on this large image. My computer does not have enough RAM to load the entire image and manipulate the pixels in this way, so I think I need to access the image data on the hard drive and somehow manipulate it so that anyone knows how to do this? thanks for helping me :)