php下载服务器端文件（禁止使用ajax提交请求）

$file_name = $dir . $name;
if (!file_exists($file_name)) {
    echo "<script>alert('文件不存在')</script>";
    exit;
} else {
    $file = fopen($file_name, "r");
    Header("Content-type: application/octet-stream");
    Header("Accept-Ranges: bytes");
    Header("Accept-Length: " . filesize($file_name));
    Header("Content-Disposition: attachment; filename=" . $name);
    echo fread($file, filesize($file_name));
    fclose($file);
    exit();
}