The IBM developerWorks site has posted a new tutorial (written up by Vikram Vaswani) about integrating your application with the Google Calendar web service.
Google Calendar allows Web application developers to access user-generated content and event information through its REST-based Developer API. PHP's SimpleXML extension and Zend's GData Library are ideal for processing the XML feeds generated by this API and using them to build customized PHP applications. This article introduces the Google Calendar Data API, demonstrates how you can use it to browse user-generated calendars; add and update calendar events; and perform keyword searches.
As mentioned, the tutorial steps you through a pretty complete interface with the Google Calendar API - everything from fetching events, adding new ones and searching your current events. He uses the GCal library to interface with the API.