session_start();
...
$_SESSION['domaine'] = $this->compl_domain;

$this->msg = "<img src='valider.jpg'> Le nom de domaine: <b>".$this->compl_domain."</b>"
  . "<font color='#008000'>est disponible</font>.<br>"
  . "<form action='commander_domaine.php?domaine=".$this->compl_domain."' method='post'>"
  . "<input src='commander.jpg' type='submit'>"
  . "</form>";

$this->msg = "<img src='valider.jpg'> Le nom de domaine: <b>".$this->compl_domain."</b>"
  . "<font color='#008000'>est disponible</font>.<br>"
  . "<form action='commander_domaine.php' method='post'>"
  . "<input type='hidden' name='domaine' value='".$this->compl_domain."' />"
  . "<input src='commander.jpg' type='submit'>"
  . "</form>";

$this->msg = "<img src='valider.jpg'> Le nom de domaine: <b>".$this->compl_domain."</b> <font color='#008000'>est disponible</font>.<br><form action='commander_domaine.php' method='post'><input src='commander.jpg' type='submit'></form>";

$valeur = $_GET['nom_du_champ'];

$valeur = (isSet($_GET['nom_du_champ']) ? $_GET['nom_du_champ'] : 'valeur_par_défaut';

<?php
session_start();
?>

<?php

class Whois_domain {
	
	var $possible_tlds;
	var $whois_server;
	var $free_string;
	var $whois_param;
	var $domain;
	var $tld;
	var $compl_domain;
	var $full_info;
	var $msg;
	var $info;
	var $os_system = "linux"; // switch between "linux" and "win"
	 
	function Whois_domain() {
		$this->info = "";
		$this->msg = "";
	}
	function process() {
		if ($this->create_domain()) {
			if ($this->full_info == "yes") {
				$this->get_domain_info();
			} else {
				if ($this->check_only() == 1) {
					$this->msg = "<img src='valider.jpg'> Le nom de domaine: <b>".$this->compl_domain."</b> <font color='#008000'>est disponible</font>.<br><form action='commander_domaine.php' method='post'><input src='commander.jpg' type='submit'></form>";
					return true;
				} elseif ($this->check_only() == 0) {
					$this->msg = "<img src='erreur.jpg'> Le nom de domaine: <b>".$this->compl_domain."</b> <font color='#FF0000'>est indisponible</font>.";
					return false;
				} else {
					$this->msg = "There was something wrong, try it again.";
				}
			}
		} else {
			$this->msg = "Only letters, numbers and hyphens (-) are valid!";
		}
	}
	function check_entry() {
		if (preg_match("/^([a-z0-9]+(\-?[a-z0-9]*)){2,63}$/i", $this->domain)) {
			return true;
		} else {
			return false;
		}
	}
	function create_tld_select() {
		$menu = "<select name=\"tld\" style=\"margin-left:0;\">\n";
		foreach ($this->possible_tlds as $val) {
			$menu .= "  <option value=\"".$val."\"";
			$menu .= (isset($_POST['tld']) && $_POST['tld'] == $val) ? " selected=\"selected\">" : ">";
			$menu .= $val."</option>\n";
		}
		$menu .= "</select>\n";
		return $menu;
	}
	function create_domain() {
		if ($this->check_entry()) {
			$this->domain = strtolower($this->domain);
			$this->compl_domain = $this->domain.".".$this->tld;
			return true;
		} else {
			return false;
		}
	}
	function check_only() {
		$data = $this->get_whois_data();
		if (is_array($data)) {
			$found = 0;
			foreach ($data as $val) {
				if (eregi($this->free_string, $val)) {
					$found = 1;
				} 
			}
			return $found;
		} else {
			$this->msg = "Error, please try it again.";
		}
	}
	function get_domain_info() {
		if ($this->create_domain()) {
			$data = ($this->tld == "nl") ? $this->get_whois_data(true) : $this->get_whois_data();
			if (is_array($data)) {
				foreach ($data as $val) {
					if (eregi($this->free_string, $val)) {
						$this->msg = "Le nom de domaine: <b>".$this->compl_domain."</b> is free.";
						$this->info = "";
						break;
					}
					$this->info .= $val;
				}
			} else {
				$this->msg = "Error, please try it again.";
			}
		} else {
			$this->msg = "Only letters, numbers and hyphens (-) are valid!";
		}
	}
	function get_whois_data($empty_param = false) { 
	// the parameter is new since version 1.20 and is used for .nl (dutch) domains only
		if ($empty_param) {
			$this->whois_param = "";
		}
		if ($this->tld == "de") $this->os_system = "win"; // this tld must be queried with fsock otherwise it will not work
		if ($this->os_system == "win") {
			$connection = @fsockopen($this->whois_server, 43);
			if (!$connection) {
				unset($connection);
				$this->msg = "Can't connect to the server!";
				return;
			} else {
				sleep(2);
				fputs($connection, $this->whois_param.$this->compl_domain."\r\n");
				while (!feof($connection)) {
					$buffer[] = fgets($connection, 4096);
				}
				fclose($connection);
			}
		} else {
			$string = "whois -h ".$this->whois_server." \"".$this->whois_param.$this->compl_domain."\""; 
			$string = str_replace (";", "", $string).";";
			exec($string, $buffer);
		}
		if (isset($buffer)) {
			//print_r($buffer);
			return $buffer;
		} else {
			$this->msg = "Can't retrieve data from the server!";
		}
	}
}
?>