Totaling #s and having it show in a text box

I'm having trouble with a form where I need to have 2 numbers (that the user enters ) multiply and then have the total or sum show in the Sub Total text box.

I need to know how to do this...
Here's what I have so far:

<input id="Mooring_Sub_Total" name="Mooring_Sub_Total" type="text" value="
<?php
echo(floatval((isset($_POST["txtSUMMER_Moor_LOA_Boat"]))?$_POST["txtSUMMER_Moor_LOA_Boat"]:"" * (isset($_GET["txtSummer_Moor_Price"]))?$_POST["txtSummer_Moor_Price"]:""))?>"
class="formTextfield_Medium" tabindex="36" onBlur="hideServerError('Mooring_Sub_Total_ServerError');">
</span>

I know this is something that the ecart coulb pbrobably do but have the rest of the form working I don't want to have to re-create it in another (new) program.

Thanks for the help in advance

you would need to hand code a javascript function to accomplish this.

here a couple pages i found through google that have details on how to accomplish this sort of thing:

Lesson06.htm
_INPUT_onChange.html

Thanks,
I'll give them a try