This is a tiny, bare minimal Google Cloud Print PHP client that can discover a printer, print a PDF, accept an invitation to a shared printer, and that's about it. It uses a Service Account for authentication.
It's got no error handling and is only here because I needed one for a tiny project of mine to print my morning crossword out. However, given the paucity of information on using Google Cloud Print, especially with a service account, I figured I might make it public in case it helped anyone else along the way.
If you're looking for a real Google Cloud Print library, you probably want to check out net-tools/google-api, among others.