第一段是读文件，下载。
第二段是字符串下载。
<?php
$fileName = "prefs.csv";
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename=' . $fileName);
header('Content-Transfer-Encoding: binary');
header('Content-Length: ' . filesize($fileName));
readfile($fileName);
<?php
$fileName = "pref_" . date("YmdHis") . ".csv";
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename=' . $fileName);
echo $csv;