http header下载文件

$filename = basename($file);
$file_extension = strtolower(substr(strrchr($filename, "."), 1));
/**
 * This will set the Content-Type to the appropriate setting for the file
 * 设置 http header 的 Content-Type 格式
 */
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)
     * 以下是不应该下载的扩展名（敏感的东西，如PHP文件）
     */
    case "php":
    case "htm":
    case "html":
    /**
     * case "txt": die("Cannot be used for ". $file_extension ." files!"); break;
     * 案例“TXT”：模具（“不能用于”。$file_extension.“文件！“）；break；
     */
    case "txt":
        $ctype = "text/plain";
        break;

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

// 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);
exit;