PHP Resize all images in folder

I need someone to write a script which will be run via a scheduled task that does the following:
1. Reads a single directory which contains a list of images.
2. Check if a thumbnail image exists.
3. If thumbnail doesn't exist, get the dimensions of image and determine the smaller dimension, crop image to a square that size and resize to a smaller size (assume 100x100 pixels)
4. Save image as thumb_originalfilname.jpg

The directory could end up with thousands of images so I'm open to suggestions that may improve the way this is done.

I have several other small projects for the right freelancer.

Please provide details of similar work done to give me confidence that you know what you are doing.

I have experience in PHP and could do this myself but don't have the time and I know there will be other people who could do this much quicker than I can.

Thanks

Tom