I recommend doing it as the command line version because the webserver may be down, ISPConfig may be down, etc. Also this lets you check any host you like... on any port you like.
At the very very bottom of the script look for
print($time." - Service ".$service_name[$i]." on host ".$service_host[$i].":".$service_port[$i]." is down\n\r");
Replace the print function with whatever you want to do... after all, it is GPL so you are free to modify it... have it grab a URL via curl or fopen, use the php mail() function or do a system call out to a mail utility like mail or mailm, etc.