There is a plugin for Serendipity (blog software that power this web site) to backup the database and all the file.
This plugin use a shell script to backup the file. Unfortunatly, some of ISP disable the feature to start a shell script in PHP for security.
Also I make a patch to not use the shell script and to use indeed the Tar.php class.
This patch doe'nt permit to use the exclude directory list, but with it you can make backup
To use it, just download the patched version and replace the original version in <install_directory>/plugins/serendipity_event_backup.