如何实现Google-Drive文件夹的持续更新到PHP应用程序？

I would like to implement a web-app with the following workflow:

1. The user logs-in
2. The user then selects one or multiple folders out of their Google-Drive
3. From this moment on, the folder/s are synced on the server
4. This happens continuously whether-or-not the user is actually on the application

Please note the following:

1. I'd much rather have this work on Google-Drive, but Dropbox or any other service will also do.
2. I don't mind the user having to paste a 'Shared URL' for the folder rather than choosing, however, I would like to avoid CRON if at all possible.

How do I go about it? Thanks in advance

If you don't implement a cron or a background process, you cannot implement continuos synchronization.