一个关于 PHP 输出文件 下载时可以分段输出下载的疑问？

public function exportCsv()
    {
        header("text/comma-separated-values;charset=UTF-8");
        header("Content-Disposition:attachment;filename=log.csv"); //“生成文件名称”=自定义
        header("Content-Description:File Transfer");
        header('Content-Type:text/comma-separated-values;charset=UTF-8');
        header('Cache-Control:must-revalidate,post-check=0,pre-check=0');
        header('Expires:0');
        header('Pragma:public');
        header('content-transfer-encoding:binary');
        echo "昵称, 头像, 纬度, 经度, 微信城市, 所在点, 检测点, 性别, 时间\r\n";
        $this->getSearchModel()->chunk(100, function ($logs) {
            foreach ($logs as $log) {
                echo $this->echoBy($log->nickname).',';
                echo $this->echoBy($log->avatarurl).',';
                echo $this->echoBy($log->latitude).',';
                echo $this->echoBy($log->longitude).',';
                echo $this->echoBy($log->city).',';
                echo $this->echoBy($log->point_city).',';
                echo $this->echoBy($log->point).',';
                echo $this->echoBy($log->sex).',';
                echo $this->echoBy($log->created_date)."\r\n";
            }
        });
    }Copy