You'll probably have to write your own; most packages like PHPNuke are overkill for your application and if you try to recycle the code you'll go blind (poor coding practices, very bad data modeling).
One of my staff, a competent Perl coder, learned PHP and wrote a content-management system similar to what you describe in two or three days. I believe he's actually using HTTP BASIC authentication on Apache, but using PHP to manage the process of creating users.
The CMS he created allows our news service and our 30 or so newspapers to exchange news articles, graphics, Quark files, et cetera. It isn't a workflow system but rather a content-sharing repository and is fairly simple in terms of design.