php 利用header 函数 下载各种文件

http://www.php.net/manual/en/function.readfile.php

<?php
/**
* 下载文件
* header函数
*
*/

dl_file($_GET ['filename']);

function dl_file($file)
{
    $file = ".//images//" . $file;
    //First, see if the file exists

    if (! is_file ( $file ))
    {
        die ( "<b>404 File not found!</b>" );
    }

    // Gather relevent info about file
    $len = filesize ( $file );
    $filename = basename ( $file );
    $file_extension = strtolower ( substr ( strrchr ( $filename, "." ), 1 ) );

    // This will set the Content-Type to the appropriate setting for the file
    switch ($file_extension)
    {
        case "pdf" :
            $ctype = "application/pdf";
            break;
        case "exe" :
            $ctype = "application/octet-stream";
            break;
        case "zip" :
            $ctype = "application/zip";
            break;
        case "doc" :
            $ctype = "application/msword";
            break;
        case "xls" :
            $ctype = "application/vnd.ms-excel";
            break;
        case "ppt" :
            $ctype = "application/vnd.ms-powerpoint";
            break;
        case "gif" :
            $ctype = "image/gif";
            break;
        case "png" :
            $ctype = "image/png";
            break;
        case "jpeg" :
        case "jpg" :
            $ctype = "image/jpg";
            break;
        case "mp3" :
            $ctype = "audio/mpeg";
            break;
        case "wav" :
            $ctype = "audio/x-wav";
            break;
        case "mpeg" :
        case "mpg" :
        case "mpe" :
            $ctype = "video/mpeg";
            break;
        case "mov" :
            $ctype = "video/quicktime";
            break;
        case "avi" :
            $ctype = "video/x-msvideo";
            break;

        // The following are for extensions that shouldn't be downloaded
        // (sensitive stuff, like php files)
        case "php" :
        case "htm" :
        case "html" :
        case "txt" :
            die ( "<b>Cannot be used for " . $file_extension . " files!</b>" );
            break;

        default :
            $ctype = "application/force-download";
    }

    $file_temp = fopen ( $file, "r" );

    // Begin writing headers
    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" );
    // Use the switch-generated Content-Type
    header ( "Content-Type: $ctype" );
    // Force the download
    $header = "Content-Disposition: attachment; filename=" . $filename . ";";
    header ( $header );
    header ( "Content-Transfer-Encoding: binary" );
    header ( "Content-Length: " . $len );

    //@readfile ( $file );
    echo fread ( $file_temp, filesize ( $file ) );
    fclose ( $file_temp );

    exit ();
}

?>