无法在DataTable.js jquery插件中的服务器端处理中加载数据

Question

I just started learning DataTable.js and I've got one doubt regarding server-side processing in DataTable.js, that is, how can I load the table in server-side processing?

I'm posting my code here so it would be easy to understand

table.html

<table id="empTable">
  <thead>
    <th>First Name</th>
    <th>Last Name</th>
    <th>Email</th>
    <th>Phone Number</th>
  </thead>
</table>

script.js

<script type="text/javascript">
  var empTable;
  $(document).ready(function(){
    empTable = $("#empTable").dataTable({
      'processing':true,
      'serverside':true,
      'ajax': 'fetchRecord.php'
    });
  });
</script>

fetchRecord.php

<?php
$conn = new mysqli('localhost','root','','example');
if($conn->connect_error) die("Could not connect".$conn->connect_error);

$fetchQuery = "SELECT firstname, lastname, email, phonenumber FROM employee";

$output = array('data' => array());

$result = $conn->query($fetch);
if($result->num_rows > 0){
  while($row = $result->fetch_assco()){
    $output['data'][] = $row;
  }
}

$conn->close();

echo json_encode($output);
?>

Please tell me what's wrong with this code and how can I fetch data from php page and load it in DataTable?

Apologies for any mistakes.

Thank you in advance