"php dropdow menu ": update a second list select from first choise

Hello everyone; thankyou for this forum.
I create a DB "koko" and one table "local" witch contains two colomns , nomlocal and NomBatiment, on total 500 records--> nomlocal and 10 records NomBatiment which are repeated for other local.
if someone can help me to create a menu dropdown when i select NomBatiment on the first select menu on the second can see just local.NomBatiment=local.nonlocal "second menu autoupdate from first select";

Thanks thanks.....

------------------



<SCRIPT language=JavaScript>

function reload(form){

var val=form.cat.options[form.cat.options.selectedIndex].value; 

self.location='local.php?cat=' + val ;

}

</script>







<?php 





$db5 = mysql_connect('localhost', 'root', '');

mysql_select_db('koko',$db5);



	$req2 = mysql_query("SELECT NomBatiment FROM local group by NomBatiment");

	

	

	$batiment="";

	

	while ($row = mysql_fetch_array($req2))

	{

	

	$nom=$row["NomBatiment"];

	$batiment.="<OPTION VALUE=\"$nom\">".$nom.'</option>';

	

	

	}

		?>

		

	<select NAME=NomBatiment onchange=\"reload(this.form)\"><option VALUE=0><?php echo $batiment?> </select>

	

<?php



$db3 = mysql_connect('localhost', 'root', '');



mysql_select_db('koko',$db3);



	$req5 = mysql_query("SELECT nomlocal FROM local");

	

	

	$local5="";

	

	while ($row5 = mysql_fetch_array($req5))

	{

	

	$nom3=$row5["nomlocal"];

	

	$local5.="<OPTION VALUE=\"$nom3\">".$nom3.'</option>';

	

	

	}

		?>

		

	<select NAME=nomlocal><option VALUE=0><?php echo $local5?></select>

thanks.....