php如何导出大数据保存到csv文件？

set_time_limit(0);
       ini_set ('memory_limit', '256M');

       $db = $this->load->database('default',true);


       $sql = "SELECT * FROM `t_mobile_number_section` $condition";

       $query = $db->query($sql);

       $result = $query->result_array();

       $filename = 'number'.time().'.csv';

       header('Content-Type: application/vnd.ms-excel');
       header('Content-Disposition: attachment;filename="'.$filename.'"');
       header('Cache-Control: max-age=0');

       // 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
       $head = array($title['number_section'],$title['area_code'],$title['province'],$title['city']);
       foreach ($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', 'gbk', $v);
       }

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

       // 计数器
       $cnt = 0;
       $limit = 100000;

       // 从数据库中获取数据，为了节省内存，不要把数据一次性读到内存，从句柄中一行一行读即可

       $i = 2;

       $count = 0;
       foreach ($result as $key => $val) {
           $count ++;
           $cnt ++;

           //每隔$limit行，刷新一下输出buffer，不要太大，也不要太小 ,大数据量时处理
           if ($limit == $cnt) { //刷新一下输出buffer，防止由于数据过多造成问题
               ob_flush();
               flush();  //刷新buffer
               $cnt = 0;
           }


           $rows[$i] = iconv('utf-8', 'gbk', $val['number_section']);
           $rows[$i+1] = iconv('utf-8', 'gbk', $val['area_code']);
           $rows[$i+2] = iconv('utf-8', 'gbk', $val['province']);
           $rows[$i+3] = iconv('utf-8', 'gbk', $val['city']);
           fputcsv($fp, $rows);

       }