using a list to call a funtion

I am trying to use the list function to call a calculation using $_SESSION[''] in a node

I have put in this code

<p>
miles: <?php print $_SESSION['a2']; ?>
</p>

<?php
If (print $_Session['a5'] = $_form_value['one'])
$b1 = "10";
if (print $_Session['a5'] = $_form_value['two']) 
$b1 = "20";
?>
<p>

Total <?php print $_SESSION['a2']*$b1; ?> Lbs

I get the math correct for one of the values (in this case it's the $b1 = "20"; value

I want to be able to choose a value based on the drop down list selection.
any suggestions as to how I might accomplish this?