Menu Kabupaten, kecamatan dan kecamatan

Assalam,.. cara sederhana coba ini gan,.. :)
jadi setelah kabupaten dipilih maka akan tampil kecamatan terkait dengan kabupaten..
selanjutnya setelah kecamatan dipilih maka akan tampil desa terkait di kecamatan...
 
<script language="JavaScript" type="text/JavaScript">
  function showKab(){
    <?php
    $query = $this->db->query("SELECT * FROM kabupaten");
    foreach ($query->result_array() as $data) {
       $id_kabupaten = $data['id_kabupaten'];
       echo "if (document.phpmu.a.value == \"".$id_kabupaten."\")";
       echo "{";
       $query_kecamatan = $this->db->query("SELECT * FROM kecamatan where id_kabupaten='$id_kabupaten'");
       $content = "document.getElementById('kabupaten').innerHTML = \"";
       foreach ($query_kecamatan->result_array() as $data2) {
           $content .= "<option value='".$data2['id_kecamatan']."'>".$data2['nama_kecamatan']."</option>";
       }
       $content .= "\"";
       echo $content;
       echo "}\n";
    }
    ?>
  }
 
  function showDesa(){
    <?php
    $query = $this->db->query("SELECT * FROM kecamatan");
    foreach ($query->result_array() as $data) {
       $id_kecamatan = $data['id_kecamatan'];
       echo "if (document.phpmu.b.value == \"".$id_kecamatan."\")";
       echo "{";
       $query_desa = $this->db->query("SELECT * FROM desa where id_kecamatan='$id_kecamatan'");
       $content = "document.getElementById('desa').innerHTML = \"";
       foreach ($query_desa->result_array() as $data2) {
           $content .= "<option value='".$data2['id_desa']."'>".$data2['nama_desa']."</option>";
       }
       $content .= "\"";
       echo $content;
       echo "}\n";
    }
    ?>
    }
</script>
 
<?php
echo "<form action='' name='phpmu' method='POST'>
<tr><th scope='row'>Kabupaten</th>  
									<td><select name='a' onchange=\"showKab()\">
                                            <option value=''>- Pilih -</option>"; 
                                            $kabupaten = $this->db->query("SELECT * FROM rb_kabupaten");
                                            foreach ($kabupaten->result_array() as $row) {
                                                echo "<option value='$row[id_kabupaten]'>$row[nama_kabupaten]</option>";
                                            }
                                  echo "</select></td></tr>
<tr><th scope='row'>Kecamatan</th>  <td><select name='c' id='kabupaten' onchange=\"showDesa()\">
                                            <option value=''>- Pilih -</option>
                                        </select></td></tr>
<tr><th scope='row'>Desa</th>       <td><select name='c' id='desa'>
                                            <option value=''>- Pilih -</option>
                                        </select></td></tr>
</form>";