$arr = array(
            1000000001,
            1000000002,
            1000000003,
            1000000004,
            1000000005
        );

$total = 0;
array_walk($arr, 'totalize');
echo '*' . $total;

function totalize( $arr_item ) {
    global $total;
    $total += $arr_item;
}

$arr = array(
            1000000001,
            1000000002,
            1000000003,
            1000000004,
            1000000005
        );

$total = 0;
array_walk($arr, 'totalize', $total);
echo '*' . $total;

function totalize( $arr_item, $key, &$total ) {
    $total += $arr_item;
    echo $arr_item . '.' . $key . '.' . $total . '<br />';
}

1000000001.0.1000000001
1000000002.1.2000000003
1000000003.2.3000000006
1000000004.3.4000000010
1000000005.4.5000000015
*0