php readfile 下载大文件失败

大文件有200多M，只下载了200K就提示下载完成，且不报错。

原因是PHP内存有限制，需要改为按块下载，就是把大文件切块后逐块下载。

if (file_exists($file))
{
    if (FALSE!== ($handler = fopen($file, "r")))
    {
        header("Content-Description: File Transfer");
        header("Content-Type: application/octet-stream");
        header("Content-Disposition: attachment; filename=".basename($file));
        header("Content-Transfer-Encoding: chunked"); //changed to chunked
        header("Expires: 0");
        header("Cache-Control: must-revalidate, post-check=0, pre-check=0");
        header("Pragma: public");
        //header("Content-Length: " . filesize($file)); //Remove

        //Send the content in chunks
        while(false !== ($chunk = fread($handler,4096)))
        {
            echo $chunk;
        }
    }
    exit;
}
echo "<h1>Content error</h1><p>The file does not exist!</p>";