function dl_file($file){
    //这是下载文件的函数,$file是文件路径.
    //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);     //则返回文件大小的字节数。若失败，则返回 false 并生成一条 E_WARNING 级的错误。
    $filename = basename($file);    //返回路径中的文件名部分。
    //图片的格式（JPG）换成小写    strtolower（把所有大写换成小写）
    $file_extension = strtolower(substr(strrchr($filename,"."),1)); //strrchr() 函数（在php中）查找字符在指定字符串中从左面开始的最后一次出现的位置，如果成功，返回该字符以及其后面的字符，如果失败，则返回 NULL。

    //This will set the Content-Type to the appropriate setting for the file
    switch( $file_extension ) {
        case "pdf": $ctype="application/pdf"; break;    ////break是结束整个循环体，continue是结束单次循环
        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;        case "php":        case "htm":        case "html":        case "txt": die("<b>Cannot be used for ". $file_extension ." files!</b>"); break;        default: $ctype="application/force-download";    }

文件下载