php导出大数据scv

1.我系统要导出30w的数据，刚开始我使用的是：

ini_set('memory_limit', '1024M');
set_time_limit(0);

header("Content-type:text/csv");
header("Content-Disposition:attachment;filename=收款明细.csv");
header('Cache-Control:must-revalidate,post-check=0,pre-check=0');
header('Expires:0');
header('Pragma:public');

(1) $str= "订单编号,来源店铺,付款ID,付款日期,付款金额,付款方式,付款币种,单据类型,记账币种,所属法人,凭证编码
";
(2) $result = 查询30w的数据。

foreach ($result as $item) {

$str .= $item['order_no'] . ",";
$str .= $item['order_from'] . ",";
$str .= $item['payment_id'] . ",";
$str .= date('Y-m-d ', strtotime($item['payment_time'])) . ",";
$str .= $item['payment_amount'] . ",";
$str .= $item['payment_method'] . ",";
$str .= $item['payment_currency'] . ",";
$str .= $documentType[$item['document_type']] . ",";
$str .= $item['account_currency'] . ",";
$str .= $item['corporate_sn'] . ",";
$str .= $item['certificate_code'] . ",";
$str .= "
";

}

echo mb_convert_encoding($str, "CP936", "UTF-8");
unset($str);
return ;
描述：我要导出30w的大数据，(2) 一下子查询30w的数据可能会导致查询连接数据库超时   (1)  如果查询30w的数据查出来了，然后拼接到一个变量上，变量也可能会导致字符串太长而报错。

所以我们想到查询的时候进行切割，分页去查询。在字符串拼接的时候，可以转换字符串然后echo,优化代码如下：

ini_set('memory_limit', '1024M');
set_time_limit(0);

header("Content-type:text/csv");
header("Content-Disposition:attachment;filename=收款明细.csv");
header('Cache-Control:must-revalidate,post-check=0,pre-check=0');
header('Expires:0');
header('Pragma:public');

$top= "订单编号,来源店铺,付款ID,付款日期,付款金额,付款方式,付款币种,单据类型,记账币种,所属法人,凭证编码
";

echo mb_convert_encoding($top, "CP936", "UTF-8");

$count = 查询总数的sql语句。

$pre_count=50000;
//循环获取
for($i=0;$i<intval($count/$pre_count)+1;$i++){

$result = 分页查询返回结果。

foreach ($result as $item) {

$str .= $item['order_no'] . ",";
$str .= $item['order_from'] . ",";
$str .= $item['payment_id'] . ",";
$str .= date('Y-m-d ', strtotime($item['payment_time'])) . ",";
$str .= $item['payment_amount'] . ",";
$str .= $item['payment_method'] . ",";
$str .= $item['payment_currency'] . ",";
$str .= $documentType[$item['document_type']] . ",";
$str .= $item['account_currency'] . ",";
$str .= $item['corporate_sn'] . ",";
$str .= $item['certificate_code'] . ",";
$str .= "
";

echo mb_convert_encoding($str, "CP936", "UTF-8");
unset($str);

}

}
return ;