Baikal
Baïkal is a Calendar+Contacts server
vobject
:date: The VObject library for PHP allows you to easily parse and manipulate iCalendar and vCard objects
http
The sabre/http library provides utilities for dealing with http requests and responses.
cache
:handbag: Simple cache abstraction layer implementing PSR-16
event
:bell: The sabre/event library provides utilities for lightweight event-based programming
xml atom
Atom XML reader and writer
xml
sabre/xml is an XML library that you may not hate.