// creer l'id du champ extra :
function make_id(){
	// creer un hash
	$hash = array();
	foreach ($this as $cle=>$val) {
		if ($cle[0]!=='_') {
			$hash[] = $val;
		}
	}
	$this->_id = substr(md5(serialize($hash)),0,6);
}

// determiner un identifiant
function get_id(){
	if (!$this->_id) $this->make_id();
	return $this->_id;
}

// transformer en tableau PHP les variable publiques de la classe.
function toArray(){
	$extra = array();
	foreach ($this as $cle=>$val) {
		if ($cle[0]!=='_') {
			$extra[$cle] = $val;
		}
	}
	$extra['id_extra'] = $this->get_id();
	return $extra;
}