/usr/bin/sabredav21 is in php-sabre-dav-2.1 2.1.10-1ubuntu1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 | #!/bin/sh
php -S 0.0.0.0:8080 `dirname $0`/../share/php/sabre21/Sabre/sabredav.php
|