(PHP 4, PHP 5)
gethostbyaddr — Get the Internet host name corresponding to a given IP address
$ip_address
   )
   Returns the host name of the Internet host specified by
   ip_address.
  
ip_addressThe host IP address.
   Returns the host name on success, the unmodified ip_address
   on failure, or FALSE on malformed input.
  
Example #1 A simple gethostbyaddr() example
<?php
$hostname = gethostbyaddr($_SERVER['REMOTE_ADDR']);
echo $hostname;
?>