PHP not giving correct results

Hi, I am trying to get Exec-PHP to work, in testing I tried just a test global code that you can use to make sure your php is working.

<?php
$a = 1;
$b = 2;

function Sum()
{
    $GLOBALS['b'] = $GLOBALS['a'] + $GLOBALS['b'];
} 

Sum();
echo $b;
?>

Everywhere else the answer is 3 of course, but for some reason when I run it through on my WordPress site using Exec-PHP it is giving me the answer of 2.

Eventually I would like to run table data from a MySQL but if it is not even giving me the right answer for a generic global…I am not sure where the problem is.

http://wordpress.org/plugins/exec-php/