My website uses a custom php/mysql calendar. Among other possible uses, I want to be able to post an event to that calendar and have it automatically copied to the logged in users's google apps calendar. I was under the impression that would require google apps premier edition. Is that right? Basically, in order to add data to google calendar from an outside domain, is premier required?
You can access Google Apps from your domain and enjoy Google Mail, Calendar, Docs, etc. You can more users if required. Here's more information on what exactly is Google Apps.