求帮看下这段PHP下载MP4文件的有关问题，文件下载不完整

求帮看下这段PHP下载MP4文件的问题，文件下载不完整

function dl_file($file){
    $len = filesize($file);
    $filename = basename($file);
    $file_extension = strtolower(substr(strrchr($filename,"."),1));
    switch( $file_extension ) {
    case "mp4": $ctype="video/mp4"; break;
    default: $ctype="application/force-download";
    }
    header("Pragma: public");
    header("Expires: 0");
    header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
    header("Cache-Control: public"); 
    header("Content-Description: File Transfer");
    header("Content-Type: $ctype");
    $header="Content-Disposition: attachment; filename=".$filename."";
    header($header );
    header("Content-Transfer-Encoding: binary");
    header("Content-Length: ".$len);
	ob_clean();
    flush();
    readfile($file);
    exit;
}
dl_file('test.mp4');

在火狐，谷歌浏览器下载正常，IE和迅雷下载文件不完整
------解决方案--------------------
ob_clean();
flush();
readfile($file);
------解决方案--------------------
没加mime头
------解决方案--------------------
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".$len);
这俩删掉试试
------解决方案--------------------
nginx + php 环境下 没有你说的那个问题，ie和迅雷都可以正常下载 不过我的文件是flv，这样的话也只能说明是mime头出错了
我的代码如下：

function dl_file($file){
$len = filesize($file);
$filename = basename($file);
$file_extension = strtolower(substr(strrchr($filename,"."),1));
switch( $file_extension ) {
case "mp4": $ctype="video/mp4"; break;
default: $ctype="application/force-download";
}
header("Pragma: public");
header("Expires: 0");
header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header("Cache-Control: public");
header("Content-Description: File Transfer");
header("Content-Type: $ctype");
$header="Content-Disposition: attachment; filename=".$filename."";
header($header );
header("Content-Transfer-Encoding: binary");
header("Content-Length: ".$len);
ob_clean();
flush();
readfile($file);
exit;
}
dl_file('http://res.805.com/videos/霍山·上林后苑(易仟亿集团) [640x480].flv'); 只改了一个文件名而已 其他的都没变