Execution de requéte mysql php

Bonjour, merci de m'aidée a executé ce code
code
<?php

include "class/class.Cnx.php";
$c = new connection();

$dd=$_GET['debut_retro'];
$df=$_GET['fin_retro'];
$deb=strtotime($dd);
$fi=strtotime($df);
//
$jours=$_GET['jours'];
$year=date('y');
$firstday = strtotime("$year-01-01");
$endday = strtotime("$year-12-31");
//
for ($ds=$firstday;$ds<=$endday;$ds+=86400) {
if ($deb == $ds) { $req='UPDATE or_retro_session SET debut_retro="'.$deb.'" '; }

if($deb==$ds+3600)
{ $deb=$deb-3600;
$req='UPDATE or_retro_session SET debut_retro="'.$deb.'" ';
}
if($deb==$ds-3600)
{$deb=$deb+3600;
$req='UPDATE or_retro_session SET debut_retro="'.$deb.'" ';
}
for ($dt=$firstday;$dt<=$endday;$dt+=86400) {

if ($fi == $dt) { $req.=',fin_retro="'.$fi.'",jours="'.$jours.'" WHERE id_retro="1" ';
mysql_query($req); }

if($fi==$dt+3600)
{ $fi=$dt-3600;
$req.=',fin_retro="'.$fi.'",jours="'.$jours.'" WHERE id_retro="1" ';
mysql_query($req); }


if($fi==$dt-3600)
{ $fi=$dt+3600;
$req.=',fin_retro="'.$fi.'",jours="'.$jours.'" WHERE id_retro="1" ';
mysql_query($req); }


}

}
echo'
retro session ajouté!<br> <a href="retro_session.php">retour</a>';

?>

message
Notice: Undefined variable: req in C:\Program Files\EasyPHP 3.0\www\youss\ajout_retro_sess.php on line 40
retro session ajouté!
retour

merci