How can I verify if a domain is online?

[Adrian4net]

How can I verify if a domain is online , for any language html xml xhtml etc.

[Atli]

... By sending a request to said domain. When they are online, they tend to respond to requests; whereas if they are not, they tend not to.

[nilayanand]

$fp = fsockopen("www.example.com", 80, $errno, $errstr, 30);
if ($fp) 
    echo "Domain Online";
else 
   echo "Not";

fclose($fp);