implementing an API system to an Uploading system

Question

Hi,

I have a upload system , where one can upload images, I want to implement an simple API system, where they can make a POST request, and get the result in simple xml.

I currently handle the upload via a php script which has a form, where users can browse and upload the file,the script then redirects to a page where they can get the results..

please tell me some methods which can be used to create a simple API system..

Thanks