// 1 megabyte
$content_length = 1000000;
 
// download binary file
header('Content-Type: application/octet-stream');
header('Content-Length: ' . $content_length);
header('Content-Disposition: attachment; filename="somefile.bin"');
 
$bytes = 1000;
$length = $content_length / $bytes;
$str = str_repeat('.', $bytes);
$micro_seconds = 20000;
 
for ($i = 0; $i < $length; $i++) {
    echo $str;
 
    // slow the download
    usleep($micro_seconds);
}