$Sql = 'select Date$de$début from Les$dates ld, Les$formations$par$dates lfpd, Les$formations lf
          where lfpd.FK_Les$formations = lf.Num
          and lfpd.FK_Les$dates= ld.Num
          and lfpd.FK_Les$formations = "'.$UneFormation.'"
          and lfpd.FK_Les$dates = "'.$UneDate.'"';

if (isset($_POST['Nom'])){
         $UneFormation = $_GET['formation'];
         $UneDate = $_GET['date'];
}

$Sql = 'select Date$de$début from Les$dates ld, Les$formations$par$dates lfpd, Les$formations lf
          where lfpd.Les$formations = lf.Num
          and lfpd.FK_Les$dates= ld.Num
          and...';

		if(isset($_GET['date'])){
 $Sql = 'select Date$de$début from Les$dates ld, Les$formations$par$dates lfpd, Les$formations lf
            where lfpd.Les$formations = lf.Num
            and lfpd.FK_Les$dates= ld.Num';
 $Req = mysql_query($Sql) or die('Erreur SQL...<br>'.$Sql.'<br>'.mysql_error());
 $Row = mysql_fetch_array($Req);
 $date = $Row['Date$de$début'];
            echo 'la formation aura lieu le $date';
 }

if(isset($_GET['date'])){
$Sql = 'select date$de$debut from les$formations$par$dates lfpd, les$dates ld, les$formations lf
		where lfpd.FK_Les$formations = lf.Num
		and lfpd.FK_Les$dates = ld.Num'
$Req = mysql_query($sql) or die('Erreur SQL !<br>'.$Sql.'<br>'.mysql_error());
$Row = mysql_fetch_array($Req);
$date = $Row['date$de$debut'];
}

SELECT ... 
   FROM table1, table2, table3
   WHERE table1.champA = table2.champZ
      AND table2.champY = table3.champQuiVaBien