Web: Interactive map with editable overlay

I take the liberty of asking this question even if I know it has already been more or less addressed in other subjects because I would like a very "powerful" system. Indeed, I am creating a website in php/javascript and I would have liked to integrate a Google Maps type map (or other but preferably Google Maps) with an overlay to highlight certain things (example: all the forests). Of course, this overlay would be created by hand by myself or other people who help me for this project. Only, I would like to integrate a permissions system on my site which would allow some people to modify this map directly on my site and that at the end of the modification, this map is updated for everyone. Is there a way to modify the map without having to change the platform (so without going through Google Earth to get a kml file for example...)? Do you know a way to do that? I don't want to provide any code since the map integration part has not been done yet. I just wanted to know if what I'm thinking about is feasible or not before I start the concrete project.

Thanks in advance