Setup Failure Codes/PDO PostgreSQL drivers
The page identifies a critical configuration problem. DAViCal will not work at all until this is resolved.
DAViCal uses a PostgreSQL database for storage.
These drivers provide backend support for PostgreSQL access via the "PHP Data Objects" (i.e. "PDO") functions.
People frequently naively request versions of DAViCal which support MySQL, probably because they have heard of it, and they haven't heard of PostgreSQL. The author of DAViCal firmly believes that PostgreSQL provides better robustness, adherence to standards, scalability and features than does MySQL. Not to mention that it has some kick-ass date handling, which is kind of useful for a calendaring server.
Nevertheless, DAViCal has been converted to use database independent libraries for querying the actual data, and the developers are willing to accept patches which start to provide support for SQLite, MySQL and other databases but none have been forthcoming. If someone cared enough to pay us we'd probably even do it ourselves...
Meanwhile, PostgreSQL goes on quietly supporting many sophisticated multi-terabyte databases around the globe.