Popular WebDAV framework for PHP. Use it to create WebDAV, CalDAV and
CardDAV servers. It supports a wide range of internet standards related to
these protocols. The most relevant are:

WebDAV
CalDAV
CardDAV
vCard 2.1, 3.0, 4.0 and jCard
iCalendar 2.0 and jCal
current-user-principal
Extended MKCOL
WebDAV-sync
CardDAV directories
CalDAV delegation
CalDAV sharing

WWW: http://sabre.io/