PHP 导出excel 数据量大时

public function ceshiexcel1(){
  set_time_limit(0);
 $filename = '病毒日志';
 header('Content-Type: application/vnd.ms-excel');
 header('Content-Disposition: attachment;filename="'.$filename.'.csv"');
 header('Cache-Control: max-age=0'); 

 //原生链接mysql
 //数据库配置
        $mysql_conf = array('host' => '127.0.0.1:52971','db' => 'center', 'db_user' => 'root', 'db_pwd' => '');
        $mysql_conn = @mysql_connect($mysql_conf['host'], $mysql_conf['db_user'], $mysql_conf['db_pwd']);
        if (!$mysql_conn) {
            die("could not connect to the database:\n" . mysql_error());//诊断连接错误
        }
        $select_db = mysql_select_db($mysql_conf['db']);
        if (!$select_db) {
            die("could not connect to the db:\n" .  mysql_error());
        }

        $sql = M('logvirus')
                    ->alias('a')
                    ->fetchsql()
                    ->field('a.id,a.strVirusName,c.address,c.rmaker,a.nVirusType,a.nunknowvirustype,a.strVirusPath,a.VirusTime,a.nunknownflag,a.terminal,d.name as rname,(case when a.iHigh_DiskType=0  then a.iLow_DiskType else a.iHigh_DiskType  end) as idisktypes')//f.iDiskTypes,j.name as iname
                     ->join('LEFT JOIN sysinfo c ON c.terminal = a.terminal')
                    ->join('LEFT JOIN pm_dealresult d ON d.mask = a.nDealResult')
                   // ->where($where)
                    ->order("a.VirusTime desc")
                    ->limit('1,10')
                    ->select(); 

        $head =  array(
                    'id'=>'编号',
                    'strvirusname'=>'病毒名称',
                    'address'=>'终端IP',
                    'rmaker'=>'别名',
                    'nname'=>'病毒类型',
                    'iname'=>'威胁来源',
                    'strviruspath'=>'病毒路径',
                    'rname'=>'处理结果',
                    'virustime'=>'时间' );

        $stmt = mysql_query($sql);
        /*echo "<pre/>";
        while ($row = mysql_fetch_array($stmt,MYSQL_ASSOC)) {
            foreach ($row as $i => $v) { 

                $row[$i] = iconv('utf-8', 'gb2312', $v);
                 var_dump($row);
        }
        }
        exit;*/
        // 打开PHP文件句柄，php://output 表示直接输出到浏览器
         $fp = fopen('php://output', 'a');
        // 输出Excel列名信息   

        //$head = array('id', 'casenum', 'terminal', 'strVirusMd5','nVirusType','nVirusCode','nOldVirusCode');
        foreach ($head as $i => $v) {
            // CSV的Excel支持GBK编码，一定要转换，否则乱码
            $head[$i] = iconv('utf-8', 'gb2312', $v);
        } 

        // 将数据通过fputcsv写到文件句柄
        fputcsv($fp, $head);  

        // 计数器
        $cnt = 0;
        // 每隔$limit行，刷新一下输出buffer，不要太大，也不要太小
        $limit = 100000;  

        // 逐行取出数据，不浪费内存
        $data = array();
        while ($row = mysql_fetch_array($stmt,MYSQL_ASSOC)) {
            $cnt ++;
            if ($limit == $cnt) { //刷新一下输出buffer，防止由于数据过多造成问题
                ob_flush();  //释放
                flush();
                $cnt = 0;
            }  

            foreach ($row as $i => $v) {
                //$row[$i] = iconv('utf-8', 'gb2312', $v);
                  $data['id'] = $cnt;
                 $data['strvirusname'] = iconv('utf-8', 'gb2312', $row['strVirusName']);
                 $data['address'] = iconv('utf-8', 'gb2312', $row['address']);
                 $data['rmaker'] =iconv('utf-8', 'gb2312', $row['VirusTime']);
                 $data['nname'] =iconv('utf-8', 'gb2312', $row['VirusTime']);
                 $data['iname'] =iconv('utf-8', 'gb2312', $row['VirusTime']);
                 $data['strviruspath'] =iconv('utf-8', 'gb2312', $row['VirusTime']);
                 $data['rname'] =  $row['rname'] ;
                 $data['virustime'] =iconv('utf-8', 'gb2312', $row['VirusTime']);
        }
        fputcsv($fp, $data);
    }
    //关闭数据库
     mysql_close($mysql_conn);  

}