If you can get the IP Address of your webhost you can do a simple ping like function.
(in fact to get the site ip address you can run ping on your domain name)
Once you have that a simple function
function serverStatus() {
if ($fp = @fsockopen("192.168.2.1", 80, $errno, $err,2)) {
return true;
} else {
return false;
}
}
You will need to change the 192.168.2.1 to the Ip address of the Site. If it returns true the server is up if false the server is down.