essaye en forcant la valeur :
/*$service =$_POST['nom_service'];
if(get_magic_quotes_gpc())
$service=stripslashes($service);
}*/
$service = "CONCESSION de DISTRIBUTION PUBLIQUE d\'argent";
var_dump($service);
$service=mysql_real_escape_string($service);
var_dump($service);
$s = "SELECT DISTINCT id_service FROM phpgw_workf_services WHERE nom_service = '$service'";
var_dump($s);
$results = mysql_query($s);
$ser=mysql_fetch_array($results, MYSQL_ASSOC);
$idservice=$ser['id_service'];