Два связанных селекта

 
$query = "SELECT * FROM `region` ORDER BY name";
$res = dbQuery($query);
 
            <select id="sel" name="region" onChange="selChange(this.form)">
                     <option value='null'>- Выберите область -</option>
                     <?php 
                           while($row = mysql_fetch_assoc($res))
                           echo "<option value='" . $row['region'] . "'>" . $row['name'] . "</option>\r\n";
                     ?>
            </select>
 
            <strong>Город:</strong><br/>
            <select name="city" style="width:300px">
                    <option value='null'>- Выберите город -</option>
                    <?php 
$query_c = "SELECT * FROM `cities` WHERE `region`='".$reg."' ORDER BY name";
$res_c =  dbQuery($query_c);
                         while($row = mysql_fetch_assoc($res_c))
                         echo "<option value='" . $row['city'] . "'>" . $row['name'] . "</option>\r\n";
                     ?>
             </select>

function selChange(seln) { 
selNum = seln.region.selectedIndex; 
Isel = seln.region.options[selNum].value; 
 
$reg=seln.region.options[selNum].value; alert($reg);
}