使用Ajax Json给出错误的Select查询php

// 前端代码
$.ajax({
  url: 'query.php',
  type: 'POST',
  dataType: 'json',
  data: { query: 'SELECT * FROM table' },
  success: function(response) {
    // 处理返回的数据
    console.log(response);
  },
  error: function(xhr, status, error) {
    // 处理错误
    console.log(error);
  }
});

// 后端代码（query.php）
<?php
// 连接数据库
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
  die("连接失败: " . $conn->connect_error);
}

// 执行查询
$query = $_POST['query'];
$result = $conn->query($query);

// 处理查询结果
if ($result) {
  $data = array();
  while ($row = $result->fetch_assoc()) {
    $data[] = $row;
  }
  echo json_encode($data);
} else {
  echo json_encode(array('error' => '查询失败'));
}

// 关闭数据库连接
$conn->close();
?>