If you want to a simple way to serve a file for download in PHP you can use this function:

function serve_file($filepath) {
    $filename = basename($filepath);

    if (headers_sent()) {
        die("Cannot output file because output already started");
    }

    $mime_type = mime_content_type($filepath);
    header("Content-type: $mime_type");
    header("Content-Disposition: attachment; filename=\"$filename\"");

    readfile($filepath);

    exit(0);
}