Upload and Resize an Image

You have created a cool contact directory and you want to allow people to upload their own photos, or you want to create an image repository which you upload images and create thumbnails out of them. Whatever it is you need to be able to upload images and resize them afterwards.

PHP has the ability to upload files such as documents or images using the multipart/form-data protocol, but how do you use this and how do you resize the images after they are uploaded.