header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename="'.basename($file_path).'"');
header('Expires: 0');
header('Cache-Control: must-revalidate');
header('Pragma: public');
header('Content-Length: ' . filesize($file_path));
readfile($file_path);
exit;

Reference:

• http://php.net/manual/en/function.readfile.php
• http://webdesign.about.com/od/php/ht/force_download.htm