php_file_download.php

     <?php

     //  do all your processing of info received to determine which file to send, etc..

     $file = '/path/to/file/to/download.txt';

     $filesize = filesize($file);

     header('Content-Description: File Transfer');

     header("Content-type: application/forcedownload");

     header("Content-disposition: attachment; filename=\"filename to display.txt\"");

     header("Content-Transfer-Encoding: Binary");

     header('Cache-Control: must-revalidate, post-check=0, pre-check=0');

     header('Pragma: public');

     header("Content-length: ".$filesize);

     ob_clean();

     flush();

     readfile("$file");

     exit;

     ?>