the problem with "beep" is that you have to be standing/sitting next to the system to hear the beep.
Beeps suck. Instead, send an email to your cell phone (SMS messaging). Here's a stupid example I have in my /etc/rc.d/rc.local file:
echo "$HOSTNAME Rebooted" | mail -s "Reboot" $myphonenumber@vtext.com
This sends an email to $4myphonenumber@vtext.com (I'm a USA Verizon cell phone customer) so I know anytime the server has been rebooted for any reason.
To do this in php, you might try
<?
mail("$phnum@vtext.com",
"Host Rebooted", $HOSTNAME);
?>
I know, anytime, anywhere. Check with your cellular/pager provider for how to send an SMS message via the 'net.