In Azure Active Directory, I have registered an App, which allows users to sign in and place items in another users calendar. This app gives me a Client ID, Secret, reply URL etc. These are used within a PHP webapp, where a user signs in using their credentials, then using our own interface allows users to place items in another users calendar.
I am doing this on a development account, xxxxx.onmicrosoft.com.
Is it possible for me to deploy this app to a clients office 365/AD account? Or do I need the client to register their own app and then provide me with Client ID, Secret etc?
What you want is called a Multi Tenant application, it is one of the options in the page where you manage your application. There is a whole sample here