write a php script that will overlay 2 images on top a third one

I have a folder with a set of files in pairs - front page, back page. I need to take the pair of images and overlay it on top of a third image. I need a php script written where i specify the third image and specify the input folder and output folder and the files will be processed in batch. I will run this locally on a apache server. I have setup my Xampp. Please find attached the sample output file