I need to choose and install a server-client platform for a ~150 user base. They should be able to upload files (after logging in) through a web form and browse folders, download whatever the other users have uploaded, etc. All files would be private, no public links are generated. Some basic admin tasks would be needed too (add users, delete files). It should run over PHP + MySQL since it's gonna be deployed in a basic, third-party hosting service.
I know that FTP would be the straightforward solution but something fancier is required. The team wouldn't mind paying for a license as long as we're getting a popular, solid product.
What's your recommendation?