A CalDAV, CardDAV and WebDAV framework for PHP. Supports PHP v7 at a minimum. Fully supports a large number of standards for calendaring, contact management, and more. Will work with both Apache and Nginx. Requires Composer to build (because of course you have to compile PHP apps these days).
While some web servers implement *DAV natively (like Apache), this code implements them itself without needing that underlying support. A very simple WebDAV server in PHP is provided as a PoC.
Nice to know that it's an option, but if there are better ones out there, those are probably preferable.