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Here’s a small function that will allow you to force a file download.

/**
 * Force a file download via HTTP.
 *
 * File is required to be on the same server and accessible via a path.
 * If the file cannot be found or some other error occurs then a
 * '204 No content' header is sent.
 *
 * @param string $path Path and file name
 * @param string $name Name of file when saved on user's computer,
 *                     null for basename from path
 * @param string $type Content type header info (e.g., 'application/vnd.ms-excel')
 * @return void
 * @access public
 */
/* public static */ function download($path, $name = null, $type = 'binary/octet-stream')
{
    if (headers_sent()) {
        echo 'File download failure: HTTP headers have already been sent and cannot be changed.';
        exit;
    }

    $path = realpath($path);
    if ($path === false || !is_file($path) || !is_readable($path)) {
        header('HTTP/1.0 204 No Content');
        exit;
    }

    $name = (empty($name)) ? basename($path) : $name;
    $size = filesize($path);

    header('Expires: Mon, 20 May 1974 23:58:00 GMT');
    header('Last-Modified: ' . gmdate('D, d M Y H:i:s') . ' GMT');
    header('Cache-Control: no-store, no-cache, must-revalidate');
    header('Cache-Control: post-check=0, pre-check=0', false);
    header('Cache-Control: private');
    header('Pragma: no-cache');
    header("Content-Transfer-Encoding: binary");
    header("Content-type: {$type}");
    header("Content-length: {$size}");
    header("Content-disposition: attachment; filename=\"{$name}\"");
    readfile($path);
    exit;
}

Very easy to use, too! Here are some examples of how you might call the function:

download('./myfile.txt');

download(__FILE__, 'a file for you.php');

download('/home/you/files/spreadsheet.xml', 'ssheet_' . date('Ymd'), 'application/vnd.ms-excel');