Php xml

Bonjour, je voudrai generer un fichier xml a partir du php en visualisant l'ensemble des donnes d'une table j'arrive a acceder au done de la table mais a l'execution les donnes ne s'affiche sous le format xml ca veut le fichier xml n'est pas generer je voudrai ke m'aidez a resoudre ce probleme voici le code:
<?php
$conn=mysql_connect("192.168.0.200 ", "weexbet" , "titanweexbet");
mysql_select_db("gefadb");
$req="select * from operations";
$res=mysql_query($req,$conn);
$dom = new DomDocument("1.0", "iso-8859-1");
//$dom = new_xmldoc();
$root = $dom->createElement("racine");
$racine = $dom->appendChild($root);
while($ligne= mysql_fetch_array($res)) {
$parent = $dom->createElement("operations");
$operation=$racine->appendChild($parent);
//ajout noeud code
$node = $dom->createElement("code");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['code']);
$tmpNode->appendChild($value);
// ajout noeud agence
$node = $dom->createElement("agence");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['agence']);
$tmpNode->appendChild($value);
//ajout noeud jour
$node = $dom->createElement("jour");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['jour']);
$tmpNode->appendChild($value);
// ajout noeud h_arrivee
$node = $dom->createElement("h_arrivee");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['h_arrivee']);
$tmpNode->appendChild($value);
// ajout noeud insertion
$node = $dom->createElement("insertion");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['insertion']);
$tmpNode->appendChild($value);
// ajout noeud ticket
$node = $dom->createElement("ticket");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['ticket']);
$tmpNode->appendChild($value);
// ajout noeud guichet
$node = $dom->createElement("guichet");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['guichet']);
$tmpNode->appendChild($value);
// ajout noeud h_appel
$node = $dom->createElement("h_appel");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['h_appel']);
$tmpNode->appendChild($value);
// ajout noeud d_attente
$node = $dom->createElement("d_attente");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['d_attente']);
$tmpNode->appendChild($value);
// ajout noeud d_travail
$node = $dom->createElement("d_travail");
$tmpNode = $operation->appendChild($node);
$value = $dom->createTextNode($ligne['d_travail']);
$tmpNode->appendChild($value);
//
$xml_string = $dom->saveXML();
echo $xml_string ;

}


?>