Unfortunately I can’t think of any tutorials off the top of my head but I’m sure they exist (google “blog database backup cron” or something similar).

You basically need two things: one script (which doens’t necessarily have to be in PHP, it can be in Perl or whatever) that runs the actual backup of your database; the other is the cron job that you’ll use to program your server to run your backup script whenever needed. If the path of your backup script is /home/communicatrix/public_html/blog/backupscript.php, you basically input that path into “command to run” line for your cron tab.