Best way to add community features to an existing website
I have a website that I would like to add to it some community features. I would like the visitors to be able to sign-up, write comments and give ratings. It would also be nice (but not required) if they could also have their own profile pages and are able to interact with each other.
Is there any PHP script or online service that can help me do this?
Existing website that already runs on PHP. Some of the features will need to be on existing pages (e.g. reviews and ratings of existing items) and some other features can take up the whole main content of the page (e.g. login, user profile etc) but still will need to integrate with the rest of the site (header, footer, sidebar, styles, sessions etc).