I use ez publish 4.0 with postgresql and couldn't get webdav to work.. After inspecting the php logs, we found this error:

PHP Fatal error: Call to undefined function mysql_real_escape_string() in /var/www/ezpublish-4.0.0/lib/ezdb/classes/ezmysqldb.php on line 908

After installing the php5-mysql bindings, it all worked fine.

Steps to reproduce

install ez publish on postgresql, without the mysql bindings present.