PHP readfile()导致文件下载损坏

这是我的代码

<?php
include "db.php";    
 $id = htmlspecialchars($_GET['id']);
 $error = false;
    $conn = mysql_connect(DB_HOST,DB_USER,DB_PASSWORD);
    if(!($conn)) echo "Failed To Connect To The Database!";
    else{   
        if(mysql_select_db(DB_NAME,$conn)){
            $qry = "SELECT Link FROM downloads WHERE ID=$id";
            try{
                $result = mysql_query($qry);
                if(mysql_num_rows($result)==1){
                    while($rows = mysql_fetch_array($result)){
                        $f=$rows['Link'];
                    }
                    //pathinfo returns an array of information
                    $path = pathinfo($f);
                    //basename say the filename+extension
                    $n = $path['basename'];
                    //NOW comes the action,this statement would say that WHATEVER output given by the script is given in form of an octet-stream,or else to make it easy an application or downloadable
                    header('Content-type: application/octet-stream');
                    header('Content-Length: ' . filesize($f));
                    //This would be the one to rename the file
                    header('Content-Disposition: attachment; filename='.$n.'');
                    //Finally it reads the file and prepare the output
                    readfile($f);
                    exit();
                }else $error = true;
            }catch(Exception $e){
                $error = true;
            }
            if($error) 
            {
                header("Status: 404 Not Found");
                }
        }
  }
?>