原创函数，在多种场景下实测有效：

function downfile($downname,$fileurl){

$filename=$fileurl;
$temp=explode(".",$filename);
file_exists($fileurl) or die('file does not exist');
ob_clean();
header('Content-Description:File Transfer');
header('Content-Type:application/octet-stream');
header("Content-Transfer-Encoding:binary");
header("Content-Disposition:attachment;filename={$downname}.".end($temp));
        header('Expires:0');
        header('Cache-Control:no-cache,must-revalidate');
header("Pragma:no-cache");  
header('Content-Length:'.filesize($fileurl));
readfile($fileurl);
exit;

}

test.php

ob_start();

downfile("newname","G:\old.jpg");