Force file download on plugin activation

Question

I am writing a plugin and want to force users to download a file immediately when they activate the plugin. Here is the code that I've written so far but doesn't work.

No fatal errors but gives warning "The plugin generated 94 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin."

// Downloader
function dcg_file_downloader() {
    $file = ABSPATH . 'wp-content/uploads/sample.txt';

    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename='.basename($file));
    header('Expires: 0');
    header('Cache-Control: must-revalidate');
    header('Pragma: public');
    header('Content-Length: ' . filesize($file));
    print $file;
}

// Plugin activation hook
function dcg_activate_plugin() {
    dcg_file_downloader();
    update_option('dcg_plugin_version', '1.0');
}
register_activation_hook( __FILE__, 'dcg_activate_plugin' );

Purpose of achieving this is to backup a specific file (for eg.: .htaccess) before making any changes into it.

Also, Is it a good idea to use ABSPATH to get WordPress installation directory/path?

Answer 1

Working with headers is tricky because you have to call them before any output is sent, as described in the Documentation. So if you have any spaces before calling them could be it. Another problem can be if any of your code is causing an error, the error massage count as output so it will trigger headers already sent as described in the answer to this question.