Configuration/settings/http auth mode
If you want to force DAViCal to use HTTP Digest Authentication:
$c->http_auth_mode = "Digest";
This will force DAViCal to use Digest authentication, with some caveats.
DAViCal's implementation of Digest authentication requires all user passwords to be stored in plain text in the database. There are probably better implementations to use :-)
Another way to use Digest authentication would be to configure DAViCal to use the webserver-provided authentication details and configure your webserver to use Digest authentication against a separate user database.
Available from DAViCal version: 0.9.9.5 and later