php使用原生函数实现文件上传及下载

if ( $_FILES && $_FILES['attachment'] && $_FILES['attachment']['tmp_name']){
    $filesize = $_FILES['attachment']['size'];
    //文件上传大小限制：5M
    if($filesize > 5*1024*1024){
        exit('上传文件大小超出限制！');
    }
    //文件保存目录，如果不存在则创建之
    $uploaddir = ROOT_DIR.'/public/attachment/notice/';
    if(!file_exists($uploaddir))
        mkdir($uploaddir,0755,true);
    //获取上传文件扩展名
    $arr_file = explode('.', basename($_FILES['attachment']['name']));
    $filetype = $arr_file[count($arr_file)-1];
    //获取源文件名
    $attachment_name = basename($_FILES['attachment']['name']);
    //对文件进行重命名以避免中文字符的影响
    $fname = md5(basename($_FILES['attachment']['name']).microtime()).'.'.$filetype;
    $uploadfile = $uploaddir . $fname;
    if (move_uploaded_file($_FILES['attachment']['tmp_name'], $uploadfile)) {
        //获取文件相对路径
        $attachment = '/public/attachment/notice/'. $fname;
    } else {
        exit('文件上传失败！');
    }
}

//输入文件标签 
Header ( "Content-type: application/octet-stream" ); 
Header ( "Accept-Ranges: bytes" ); 
Header ( "Accept-Length: " . filesize ( $attachment_filepath ) ); 
Header ( "Content-Disposition: attachment; filename=" . $attachment_filename ); 
//输出文件内容 
//读取文件内容并直接输出到浏览器 
echo file_get_contents($attachment_filepath); 
exit ();