I believe cPanel will send you an email every 5 minutes if one of your monitored services is down - Under WHM -> Service Manager -> Ensure the box to monitor MySQL is selected.
With your cron - change it to;
*/20 * * * * /usr/bin/php -q /home/username/public_html/admin/mysqlstat.php
-q is quiet or not header mode (might explain why you are getting blank emails?) - and have your mysqlstat.php file generate an email if the service is down.
From LinuxHelp.net
http://www.linuxhelp.net/guides/cron/
>> /dev/null 2>&1 part means to send any standard output to /dev/null (the linux trash can) and to redirect standard error (2) to the same place as the standard output (1). Basically it runs the command without any output to a terminal etc.
Hope this helps
