There are many whois servers around the interweb, we can use such whois server to determine if an domain name is already taken or is still available to be registered. Mostly whois server are connected on port 43. Below is the php script i developed to check domains with com,net,org,info,biz and name extensions.

<?php
function checkdomain($domainname)
{
	$extension = explode(".",$domainname);
	$extension = $extension[1];
	switch($extension)
	{
		case "com": $server = "whois.verisign-grs.com"; break;
		case "net": $server = "whois.verisign-grs.com"; break;
		case "org": $server = "whois.pir.org"; break;
		case "info": $server = "whois.afilias.net"; break;
		case "biz": $server = "whois.biz"; break;
		case "name": $server = "whois.nic.name"; break;
		default: return "Given Extension not supported"; exit();
	}
    // connecting to the whois server.
    $handle = fsockopen($server, 43);
    if (!$handle)
        return false; // connectin failure 

    //asking the server
    fwrite($handle, $domainname."\r\n");

    // getting response
    $response = '';
    while (!feof($handle))
        $response .= fgets($handle, 1024);

    fclose($handle);
	//checking if domain is available or not
	if($extension=="com" || $extension=="net" || $extension=="name")
	{
		if(stripos($response, 'No match') === FALSE )
			return $domainname." is not Available!";
		else
			return $domainname. " is Available!";
	}
	else
	{
		if(stripos($response, "NOT FOUND") === FALSE)
			return $domainname. " is not Available!";
		else
			return $domainname. " is Available!";
	}
}
echo checkdomain("anildewani.com");
echo checkdomain("anildewani.net");
echo checkdomain("anildewani.org");
echo checkdomain("anildewani.info");
echo checkdomain("anildewani.biz");
echo checkdomain("anildewani.name");
echo checkdomain("anildewani.in");
?>