php强制输出到浏览器下载

            $file_name="test.mp3";
            $mp3_url = "";
            header( "Pragma: public" );
            header('Expires: 0');
            header( "Cache-Control: must-revalidate, post-check=0, pre-check=0" );
            header( "Cache-Control: private", false );
            header( "Content-Type: application/force-download ");
            //header( "Content-Length: " . $fileSize);
            header('Content-Disposition: attachment; filename="'.$file_name.'"');
            header('Content-Transfer-Encoding: binary');
            header('Connection: close');
            readfile($mp3_url);
            exit();