Rebuild Drop-down Menu ....

I have 2 drop-down menus (1) States (2) Shows By State. My question is how
do I invoke a re-build of the Shows By State drop-down when the user
requests a different state. I believe this has to be done with Javascript
and I don't know Javascript. Could someone supply a small example of how
this is done?

I am using PHP and MySQL. Following is the PHP code for building the state
drop-down menu:

<tr><td bgcolor="#EFC891" width="496" align="left"><b>Select a Show in Your
State:&nbsp;&nbsp;

<select name="ShowState" >
<?php
$stateName=getStateName();
$stateCode=getStateCode();
for ($n=1;$n<=50;$n++)
{
$state=$stateName[$n];
$scode=$stateCode[$n];
echo "<option value='$scode'";
if ($scode== $searchState)
echo " selected";
echo ">$state
";
}
?>
</select>