PHP导出MySQL到CSV文件的问题

Question

<?php
set_time_limit(0);
ini_set('memory_limit', '128M');

$fileName = date('YmdHis', time());

if(isset($_GET['mode'])){
    $mode = $_GET['mode'];
}else{
    $mode = 'down';
}
if($mode == 'down'){
    header('Content-Encoding: UTF-8');
    header("Content-type:application/vnd.ms-excel;charset=UTF-8");
    header('Content-Disposition: attachment;filename="' . $fileName . '.csv"');
    $fp = fopen('php://output', 'a');//流输出 
}else{
    $fp = fopen($fileName . '.csv', 'a');//保存到文件
}

$dbhost = '';
$dbuser = '';
$dbpwd = '';
$con = mysqli_connect($dbhost, $dbuser, $dbpwd);
if (mysqli_connect_errno()) {
   die('connect error');
}
$database = '';
mysqli_select_db($con, $database);
mysqli_query($con, "set names UTF8");

$sql = "SELECT count(id) as total FROM `zywd` where 1=1";
$result = mysqli_query($con,$sql);
$rs = mysqli_fetch_array($result);
$count = $rs['total'];
mysqli_free_result($result);

$nums = 500;//每次查询得记录数
$step = $count / $nums;
$step = ceil($step);//查询次数

for($s = 1; $s <= $step; $s++) {
    $start = ($s - 1) * $nums;
    $result = mysqli_query($con,"SELECT * FROM `zywd` where 1=1 ORDER BY `id` LIMIT {$start},{$nums}");
    if($result) {
        while($row = mysqli_fetch_assoc($result)) {
            fputcsv($fp, $row);
        }
        mysqli_free_result($result); 

        ob_flush();
        flush();
    }
}

mysqli_close($con);
fclose($fp);

使用上面的PHP代码，导出MySQL表中的15万条数据到CSV文件。
在Win10+apache+php7环境下执行，可以以流文件方式下载下来，但是保存到服务器文件会出现500错误。
在centos+nginx+php7环境下，流文件下载出现“失败 - 网络错误”，保存到服务器文件出现502错误，每次文件大小停留在12.84M。
测试导出少量数据没有问题。应该如何排查解决问题？

Answer 1

试一下把执行时间延长 ini_set("max_execution_time", "3600");