Quercus is great, although I’ve found I have to work round some of its unimplemented methods or implementation differences from PHP. I test on a plain Linux/Apache/PHP server though, and wanted a way to detect Quercus so I can conditionally use different work arounds. You can do this with the following PHP:
1) It works with WordPress multisite, so that you can set on a per site basis if a site requires login to view the content.
2) It allows you to set a minium user level to view content, so for example, only Editors and above can view the content. (This is useful when you have automatic user registration set up or on a multisite where users may have access to multiple sites.)