According to wikipedia WHOIS “WHOIS is a TCP-based query-response protocol for querying an official database in order to determine the owner of a domain name, an IP address, or an autonomous system number on the Internet”. ICAAN is presently undertaking the task for managing the WHOIS information.

For querying WHOIS servers here is the PHP code.

function check_domain($domain,$ext)
{

	/************************	SERVER DEFINITIONS	************************************/
	$serverdefs= array(
							"com" => array("whois.crsnic.net","No match for"),
							"net" => array("whois.crsnic.net","No match for"),
							"org" => array("whois.pir.org","NOT FOUND"),
							"biz" => array("whois.biz","Not found"),
							"info" => array("whois.afilias.net","NOT FOUND"),

					);
	/*********************** 	END SERVER DEFINITIONS	*********************************/

    $server = $serverdefs[$ext][0];
    $nomatch = $serverdefs[$ext][1];

	$output="";
    if(($sc = fsockopen($server,43))==false){return 2;}
           fputs($sc,"$domain.$ext\n");

	while(!feof($sc)){$output.=fgets($sc,128);}
    fclose($sc);
    //compare what has been returned by the server
    if (eregi($nomatch,$output)){
		return true; // if matched
    }else{
        return false;
    }
}

Code #04 – #13 defines the array of official WHOIS server for respective extensions of domains and the respective responses (a chunk of) we get after querying the database servers. For example if we query whois.crsnic.net we get response which contains the phrase “No match for” if the domain does not exists.
Code #19 – #23 requests the respective server on the basis of extension and gets the response in form of string on variable $output.
Then the response is matched with the respective value of response defined in the array (which contains the phrase when the domain does not exists). Then it outputs the boolean value accordingly if the domain does not exists or can be registered it returns true, false otherwise.