<? $apparecchio=intval($_GET['client_id']);
$link = mysql_connect('localhost', 'root', ''); //changet the configuration in required
if (!$link) {
    die('Could not connect: ' . mysql_error());
}
mysql_select_db('gecko');
$query="SELECT id, client_id, tipo_apparecchio, modello FROM apparecchi where $client = apparecchi.client_id";
$result=mysql_query($query);

?>


<select name="apparecchio">
<option>Seleziona Apparecchio</option>
<? while($row=mysql_fetch_array($result)) { ?>
<option value=<?=$row['id']?>><?=$row['modello']?></option>
<? } ?>
</select>