example-download.php

<?php

//
$filepath = 'file.txt';

/**
"htm" => "text/html",
"exe" => "application/octet-stream",
"zip" => "application/zip",
"doc" => "application/msword",
"jpg" => "image/jpg",
"php" => "text/plain",
"xls" => "application/vnd.ms-excel",
"ppt" => "application/vnd.ms-powerpoint",
"gif" => "image/gif",
"pdf" => "application/pdf",
"txt" => "text/plain",
"html"=> "text/html",
"png" => "image/png",
"jpeg"=> "image/jpg"
*/
$finfo = finfo_open(FILEINFO_MIME_TYPE);
header('content-type:' . finfo_file($finfo, $filepath));

//  exe --> binary
$finfo = finfo_open(FILEINFO_MIME_ENCODING);
header('content-transfer-encoding:' . finfo_file($finfo, $filepath));

//
header('content-disposition:attachment;filename="' . basename($filepath) . '"');

//
readfile($filepath);

//
exit();

//  ref:
//      http://stackoverflow.com/questions/7263923/how-to-force-file-download-with-php