PHP Variable in Shortcode

I wrote a simpe PHP script and have been trying to return the results in WordPress shortcodes, but I can’t seem to get it to work. Here is what I have:

function securetotal() {
    return $secure_total;
}
add_shortcode('secure', 'securetotal');

$secure_total is the predefined variable that I need to be displayed. I can display simple text instead of the variable, but the variable is not transferring any results. Am I missing something? Any ideas?

Help is much appreciated.