Javascript change property of PHP variable according to date in list menu selection

I'm creating a booking form which selects prices and dates from a Mysql databse according to the code transported in the URL.

I have successfully got the form to display dates for this year and next year in a drop down menu/list form item.

However, I would like to change the price according to what year the selected date falls in.

The key variable that needs to change is this $coursefee

If a date in 2009 date (e.g. 2009-10-12) is selected the varibiable $coursefee would change to be

<?php $coursefee=$row_rs_course['price09'];?>

And if a date like 2010-10-12 is selected the varibiable $coursefee would change to be

<?php $coursefee=$row_rs_course['price10'];?>

To complicate matters the menu is dyamically populated too:

<select name="dates" class="course" id="dates">
<?php
do { 
?>

<?php

$date1 = strftime("%d %b",strtotime($row_rs_dates['start_date']));                       
$date2 = strftime("%d %b %Y",strtotime($row_rs_dates['finish-date']));

?>

<option value="<?php echo($date1." - ".$date2); ?>"><?php echo($date1." - ".$date2);?></option>
                   
} while ($row_rs_dates = mysql_fetch_assoc($rs_dates));
  $rows = mysql_num_rows($rs_dates);
if($rows > 0) {
mysql_data_seek($rs_dates, 0);
$row_rs_dates = mysql_fetch_assoc($rs_dates);

</select>

I'm pretty sure the answer lies in the murky world of javascript but unfortunately I haven't the first clue about javascript.

Can anyone help me with this?

Cheers

Dave