HELP: change second select menu depending on first selection in javascript/ php

Hi all, what i want to do is when I select institution A, then the school of institution A come out at the second selection only.. when I select institution B, then the school of institution B come out at second the selection...etc
Help...Thanks~

<?php $sql = "SELECT DISTINCT i_name FROM school";
// execute query
$result = mysql_query($sql);
if(isset($_POST["InstitutionName"]))
$InstitutionName = $_POST["InstitutionName"];
echo"<select name='InstitutionName'>";
while ($row = mysql_fetch_array($result))
{ $i_name=$row["i_name"];
echo"<option value='$i_name'";
if (isset($InstitutionName) && $InstitutionName == $i_name)
echo " selected> $i_name";
else echo "> $i_name"; }
echo "</select><br>"; ?>
</td>
</tr>
<tr valign="baseline">
<td nowrap="nowrap" align="left">School Name</td>
<td>:</td>
<td>School of &nbsp;
<?php $sql = "SELECT DISTINCT s_name from school";
// execute query
$result = mysql_query($sql);
if(isset($_POST["SchoolName"]))
$SchoolName = $_POST["SchoolName"];
echo"<select name='SchoolName'>";
while ($row = mysql_fetch_array($result))
{ $s_name=$row["s_name"];
echo"<option value='$s_name'";
if (isset($SchoolName) && $SchoolName ==$s_name)
echo " selected> $s_name";
else echo "> $s_name"; }
echo "</select><br>"; ?>
</td>
</tr>

 

please use the code tag so your code is readable and second please tell us what isn't working? or what your problem realy is!

 

use onChange=.... <select name='ddlXXX' onChange="some_javascript_function(this.value)"> ... </select>