In your case, the operator MERGEwill look like this:
$DB->query("MERGE INTO periodetojour dst
USING (
SELECT '".addslashes($idperiode)."' idperiode,
'2' idjour,
'".addslashes($mardi)."' heure
) src
ON src.idperiode = dst.idperiode
WHEN MATCHED THEN UPDATE SET
dst.idjour = src.idjour,
dst.heure = src.heure
WHEN NOT MATCHED THEN INSERT (idperiode, idjour, heure)
VALUES(src.idperiode, src.idjour, src.heure)");
, idperiode - . (idperiode, idjour), ON, WHEN MATCHED THEN UPDATE SET :
ON src.idperiode = dst.idperiode
AND src.idjour = dst.idjour
WHEN MATCHED THEN UPDATE SET
dst.heure = src.heure