从服务器下载的文件（使用 download.php）将无法打开。为什么？

Question

我是初学者。我到处搜索，但不知道为什么通过客户端（Win 7 Firefox）从服务器下载文件后无法打开该文件。我尝试过 PNG 文件和 MP4 文件。下载完成，但文件无法打开。这是我的脚本；

$dl_file = $_GET['val']; //Verified the full path and the file name gets passed here
$basename = basename($dl_file);
$ext = pathinfo($dl_file, PATHINFO_EXTENSION);
$length   = sprintf("%u", filesize($dl_file));

    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename='.$basename.'');
//manually tried '.$basename.'.PNG' - DID NOT work. How to pass $ext here?
        header('Content-Transfer-Encoding: binary');
        header('Connection: Keep-Alive');
        header('Expires: 0');
        header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
        header('Pragma: public');
        header('Content-Length: ' . $length);

        set_time_limit(0);
        readfile($dl_file);

我无法想象为什么下载的文件无法打开。是否腐败？请对此进行更多说明。先感谢您。

Answer 1

经过多次尝试，我发现添加下面所示的两行代码（ob_clean和flush）（和/或添加预检查、后检查参数） - 在所有浏览器上都有效。谢谢。

if (file_exists($dl_file)) {
    header('Content-Description: File Transfer');
    header('Content-Type: application/octet-stream');
    header('Content-Disposition: attachment; filename="'.basename($dl_file).'"' );
    header('Expires: 0');
    header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
    header('Pragma: public');
    header('Content-Length: ' . filesize($dl_file));
    ob_clean();
    flush();
    readfile($dl_file);
    exit;
}