Lets see. My php is a little rusty, but I think the following might do what you need.
$strHost = gethostbyaddr($_SERVER['REMOTE_ADDR']);
In my case, this would return something like...
UNIQUEHASH-UNIQUEHASH.cpe.net.cable.rogers.com
I would imagine what you want is just the rogers or the rogers.com. You can perform some string manipulation functions on $strHost to retrieve that.