Upload file to OneDrive folder via php /which product do I / my customer need?

Hi,
I'm totally new to this topic.
I'm about to develop an app (Backend:PHP, Frontend:JS) which should be able to push files (generated in php on my server) to an OneDrive Folder of my customer. I am stuck at which product do I (for developing) and later my customer need for that.
I think I need the "client credentials flow", which MS product/account is needed for that?

Is an onedrive account enough?
Do I/my customer need an additional azure account to register the app?
Thanks for help