simple OCR in php

Hello all, I am looking for a way to incorporate some simple OCR into a php script. The user will bulk scan a pile of invoices. I want the php script to look at each invoice and read a number off the invoice. The image will then be renamed, and be organized into a directory and the file name will be added to a database. (all of these steps are straight forward once the number is read.) I have no problem with a system that requires a special OCR font and/or some sort of registration mark to help locate the Invoice number. Can anybody tell me of any tools out there that can do this? Thanks, Ray