Ciao e grazie,

posto uno script che ho trovato:

javascript

codice:
<script type="text/javascript" src="./js/ajax.js"></script>
<script type="text/javascript">
var ajax = new Array();

function creaConducenti(index)
{
	var obj = document.getElementById('clienti');
	eval(ajax[index].response);	// Executing the response from Ajax as Javascript code	
}

function getConducente(sel)
{
	var countryCode = sel.options[sel.selectedIndex].value;
	document.getElementById('dhtmlgoodies_city').options.length = 0;	// Empty city select box
	if(countryCode.length>0){
		var index = ajax.length;
		ajax[index] = new sack();
		
		ajax[index].requestFile = 'conducentiselect.php?ragione_sociale='+ragione_sociale;	// Specifying which file to get
		ajax[index].onCompletion = function(){ creaConducenti(index) };	// Specify function that will be executed after 
    //file has been found
		ajax[index].runAJAX();		// Execute AJAX function
	}
}
</script>
XHTML

Codice PHP:
<form action="" method="post">
<table>
    <tr>
        <td><select id="clienti" name="clienti" onchange="getConducente(this)" multiple>
            <?php 
      $exp = mysql_query("SELECT ragione_sociale FROM clienti ORDER BY ragione_sociale;");
      while($cliente=mysql_fetch_array($exp)){
      
            print("<option value='".$cliente["ragione_sociale"]."'>".$cliente["ragione_sociale"]."</option>");
            
            } ?>
        </select>
        </td>
    </tr>
    <tr>
<td><select id="dhtmlgoodies_city" name="dhtmlgoodies_city" multiple>
        
        </select>
        </td>
    </tr>
    <tr>
        <td><select id="flotta" name="flotta" multiple>
            <?php 
      $exp3 = mysql_query("SELECT * FROM flotta ORDER BY targa;");
      while($flotta=mysql_fetch_array($exp3)){
      
            print("<option value='".$flotta["targa"]."'>".$flotta["targa"]." - ".$flotta["modello"]." - ".$flotta["versione"]."</option>");
            
            } ?>
        </select>
        </td>
    </tr>    
</table>
</form>
file conducentiselect

Codice PHP:
$exp2 = mysql_query("SELECT * FROM conducenti WHERE ragione_sociale = '".$_GET["ragione_sociale"]."';");
      while($conducente=mysql_fetch_array($exp2)){
      
            print("<option value='".$conducente["conducente"]."'>".$conducente["conducente"]."</option>");
            
            }                                  
?> 
dov'è che sbaglio?