$REMOTE_HOST or $_SERVER['REMOTE_HOST'] will give you the reversed ip if apache is setup to do hostname lookups.

It looks like your server is not set up correctly.

 

REMOTE_HOST' The Host name from which the user is viewing the current page. The reverse dns lookup is based off the REMOTE_ADDR of the user.

Note: Your web server must be configured to create this variable. For example in Apache you'll need HostnameLookups On inside httpd.conf for it to exist. See also gethostbyaddr().