Untitled

How to alter headers to download a file in WordPress when headers are already being sent out in a previous file?
$tmp_handle = fopen('php://memory', 'r+');

foreach ($arr as $fields) {
    fputcsv($tmp_handle, $fields);
}

header('Content-type: text/csv');
header('Content-disposition: attachment;filename=MyVerySpecial.csv');
rewind($tmp_handle);

echo stream_get_contents($tmp_handle);

add_action('template_redirect', 'download_page');
function download_page() {
    if($_GET['download'] == 'true') {
        //Output header information and the file content
        exit; // Stops Wordpress from executing anything else.
    }
}