代码：

function downloadFile(){
    //文件路径（路径+文件名）
    $file_name = U('/temp/downloadfile.xlsx','',false,true);
// required
header('Pragma: public');
//no cache
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Cache-Control: private',false);
//强制下载
header('Content-Type:application/force-download');
header('Content-Disposition: attachment; filename="'.basename($file_name).'"');
    header('Content-Transfer-Encoding: binary');
    header('Connection: close');
    //输出到浏览器
    readfile($file_name);
    exit();
}

代码2：

//文件路径
$filename = U('/temp/downloadfile.xlsx','',false,true);

header("Content-Type:application/force-download");
header("Content-Disposition:attachment;filename=".basename($filename));
readfile($filename);