global variables in theme's function.php

Hi,
I am writing a customer theme, and want to use a global variable in the function.php.
It looks like the global variable is loosing it value which is set in a filter function.

How can I achieve that the value, which is set in the filter function is available in the function.php?

Here is the code of my function.php showing the issue I have.

<?php

$myGlobalVar;
print_r($myGlobalVar);

add_theme_support('menus');

//register menue locations
function register_my_menus() {
    global $helperClass;
    register_nav_menus(
            array('top-menu' => __('Top Menu'),
                'left-menu' => __('Left Menu'))
    );
}

add_action('init', 'register_my_menus');

// add hook
add_filter('wp_nav_menu_objects', 'my_wp_nav_menu_objects_sub_menu', 10, 2);

// filter_hook function to react on sub_menu flag
function my_wp_nav_menu_objects_sub_menu($sorted_menu_items, $args) {
    global $myGlobalVar;

    $myGlobalVar = array("this is a global var", "This as well");

    return $sorted_menu_items;
}

?>

thanks to all of you for helping me here.
Gerald