This sounds really interesting! If you're going to freely license it, please use “
GNU Affero General Public License 3 or any later version of the license”, because this license was designed to protect the freedom of all computer users even if the software is run on a server as a service.
Basically, I would like to learn how one can use PHP to execute Java programs on a webserver, because I'm thinking about writing freely licensed software for highly integrated, automated online publishing, and my standalone offline tools are written in Java for portability. I know both Java and PHP, but up to now I don't know how I could set up an online service. epubcheck could certainly be part of it in order to only publish valid EPUB files.
So I would highly appreciate if you would share your experiences, methods, or even code.