Since you’re on a Linux box, I would highly recommend Google’s open source project ocropus.
It’s not PHP, but I think it will be your best option. Of course you can call it from within PHP via exec. Its mature and has a lot of options. From the project site:
The OCRopus engine is based on two research projects: a high-performance handwriting recognizer developed in the mid-90’s and deployed by the US Census bureau, and novel high-performance layout analysis methods.
There is also another open source project, tesseract. I’ve used this in the past as well and have been pleased with the results. Includes training, limiting your alphabet, etc.