javascript - php mysqli fetch_all() returns json normally, but the front-end ajax cannot get json?

Question

php code

<?php
//include "db_connect.php";
$db = new MySQLi("localhost", "root", "password", "paginate");
$db->set_charset('utf8mb4');

header('Content-type: application/json;charset=utf-8');

$total = 0; //Total number of rows of table data
$per_page = 6; // Number of lines per page
$current_page = 1; // Which page is currently displayed?
$total_pages = 0; //Total number of pages

// Get the current page number
if (!empty($_GET['current_page'])) {
  $current_page = $_GET['current_page'];
}

// Get the total number of rows of table data
$query1 = 'select count(*) from user';
$row1 = $db->query($query1);
if ($row1 && $rowd = $row1->fetch_row()) {
  $total = $rowd[0];
}

// Get the total number of pages
$total_pages = ceil($total / $per_page);

// paging
$pre = ($current_page - 1) * $per_page;
$query2 = "select * from user limit $pre,$per_page";
$rows = $db->query($query2);

$arr = [];
if($rows) {
  $arr = $rows->fetch_all(MYSQLI_ASSOC);
}
$pagination = [
  'total' => $total,
  'per_page' => $per_page,
  'current_page' => $current_page,
  'total_pages' => $total_pages
];

array_push($arr, $pagination);
echo json_encode($arr, true);

postman screenshot:

js code:

 window.onload = function () {
        getData(1);
    }

    function addURLParam(url, name, value) {
        url += (url.indexOf("?") == -1 ? "?" : "&");
        url += encodeURIComponent(name) + "=" + encodeURIComponent(value);
        return url;
    }
    
    function getData(page) {
        var data = document.getElementById("data");

        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function () {
            if (xhr.readyState == 4 && xhr.status == 200) {
                var jsonText = xhr.responseText;
                console.log(JSON.parse(jsonText));
            }
        }
        var url = "paginate.php";
        url = addURLParam(url, "current_page", "page");
        xhr.open("get", url, true);
        xhr.send();
    }

There is no content in the database in front of the chrome console

Should be

$query2 = "select * from user limit $pre,$per_page";
$rows = $db->query($query2);

$arr = [];
if($rows) {
  $arr = $rows->fetch_all(MYSQLI_ASSOC);
}

There is a problem with the sql here, because there will be errors before if ($rows) is added. However, I am very surprised why php can run normally. The php version is 7.0.8

Answer 1

It’s a problem that js does not correctly pass parameters into addURLParam()

url = addURLParam(url, "current_page", "page");

changed to

url = addURLParam(url, "current_page", page);

It’s really shameful to say that I discovered this bug after I posted the question. I edited and modified the question once, but the school had a power outage and the computer ran out of power, so I couldn’t test it. . . . . .

---

But the important point is that I added the addURLParam method last. It turned out to be direct

xhr.open("get",  "paginate.php?current_page="+page, true);

addURLParam() is a method in js advanced programming, it says

An error that often occurs when using GET requests is that there is a problem with the query string format. The name and value of each parameter in the query string must be encoded using encodeURIComponent() before being placed at the end of the URL

Friends who know the reason can answer this point, I will search it first~