Ojala que les sirva:
<script>
function selectAsociado(){
var seccion
seccion = document.form1.seccion[document.form1.seccion.selectedIndex].value
if (seccion != 0) {
mis_subsecc=eval("secc_sub" + seccion)
num_seccisub = mis_subsecc.length
document.form1.subseccion.length = num_seccisub
for(i=0;i<num_seccisub;i++){
document.form1.subseccion.options[i].value=mis_subsecc[i]
document.form1.subseccion.options[i].text=mis_subsecc[i]
}
}else{
document.form1.subseccion.length = 1
document.form1.subseccion.options[0].value = "-"
document.form1.subseccion.options[0].text = "-"
}
document.form1.subseccion.options[0].selected = true
}
</script>
<?php
echo "<script>";
$result = mysql_query("SELECT id_seccion FROM secciones");
while($row = mysql_fetch_array($result)) {
echo "var secc_sub".$row[id_seccion]."= new Array('---'";
$result2 = mysql_query("SELECT subseccion FROM subsecciones WHERE id_secc = '$row[id_seccion]'");
while($row2 = mysql_fetch_array($result2)) {
echo ", '$row2[subseccion]'";
}
echo ")\n";
}
echo "</script>";
echo "<form name=\"form1\">
<table width=\"50%\">
<tr>
<td>Sección:</td>
<td>
<select name=\"seccion\" onchange=\"selectAsociado()\">\n
<option value=\"0\">[-- Seleccione --]</option>\n";
$query1 = "SELECT id_seccion, seccion FROM $TBL_SECCIONES ORDER BY seccion ASC";
$result1 = mysql_query($query1);
while ($rows1 = mysql_fetch_array($result1)){
echo "<option value=\"{$rows1[0]}\"";
if ($rows["id_seccion"] == $rows1[0]) echo " selected";
echo "\">{$rows1[1]}</option>\n";
//echo "<option value=\"{$rows1[0]}\">{$rows1[1]}</option>\n";
}
mysql_free_result($result1);
echo "</select>
</td>
</tr>
<tr>
<td>Sub Sección:</td>
<td>
<select name=\"subseccion\">
<option value=\"-\">---";
$query2 = "SELECT id_subsec, subseccion FROM $TBL_SECCIONES_SUB WHERE id_secc = '{$rows["id_seccion"]}'";
$result2 = mysql_query($query2);
while ($rows2 = mysql_fetch_array($result2)){
echo "<option value=\"{$rows2[0]}\"";
if ($rows["id_subsec"] == $rows2[0]) echo " selected";
echo "\">{$rows2[1]}</option>\n";
}
echo "</select>
</td>
</tr>
</table>
</form>";
?>