I think the reason to use define is so that it is absolutely that value and it can't be changed unless you physicially change it wherever it's taking that value from (in the script or in the database).

Take this example...

PHP Code:
<?php
define
('TEST'1);
$test 2;

echo 
TEST// outputs 1
echo '<br />';
echo 
$test// outputs 2
?>
As you can see TEST and $test both have different values. $test can be changed perhaps via the browser if it was listening for $_GET... and yet TEST will never change as long as the above script stays as it is.

No worries for the help, glad it helps you out, anytime.