PHP - Datatable In Different Color Rows.

It give an the link like 

http://localhost/aps/undefined

Object not found! The requested URL was not found on this server. The link on the referring page seems to be wrong or outdated. Please inform the author of that page about the error. If you think this is a server error, please contact the webmaster. Error 404 localhost Apache/2.4.35 (Win32) OpenSSL/1.1.0i PHP/7.2.11

//The php code to fetch data

//The php code to fetch data
<?php
include('db.php');
$query = '';
$output = array();
$query .= "SELECT * FROM users ";
if(isset($_POST["search"]["value"]))
{
    $query .= 'WHERE first_name LIKE "%'.$_POST["search"]["value"].'%" ';
    $query .= 'OR last_name LIKE "%'.$_POST["search"]["value"].'%" ';
}
if(isset($_POST["order"]))
{
    $query .= 'ORDER BY '.$_POST['order']['0']['column'].' '.$_POST['order']['0']['dir'].' ';
}
else
{
    $query .= 'ORDER BY id DESC ';
}
if($_POST["length"] != -1)
{
    $query .= 'LIMIT ' . $_POST['start'] . ', ' . $_POST['length'];
}
$statement = $connection->prepare($query);
$statement->execute();
$result = $statement->fetchAll();
$data = array();
$filtered_rows = $statement->rowCount();
foreach($result as $row)
{
    $sub_array = array();
    $sub_array[] = $row["first_name"];
    $sub_array[] = $row["last_name"];
    $sub_array[] = '<button type="button" name="update" id="'.$row["id"].'" class="btn btn-warning btn-xs update">Update</button>';
    $sub_array[] = '<button type="button" name="delete" id="'.$row["id"].'" class="btn btn-danger btn-xs delete">Delete</button>';
    $data[] = $sub_array;
}
$output = array(
    "draw"              =>  intval($_POST["draw"]),
    "recordsTotal"      =>  $filtered_rows,
    "recordsFiltered"   =>  get_total_all_records(),
    "data"              =>  $data
);
echo json_encode($output);
?>

//javasrcipt jquery
var dataTable = $('#user_data').DataTable({
        "processing":true,
        "serverSide":true,
        "order":[],
        "ajax":{
            url:"fetch.php",
            type:"POST"
        },
        "columnDefs":[
            {
                "targets":[0, 3, 4],
                "orderable":false,
            },
        ]

    });

//please dont talk about sql injection, i'll work on it, the problem with this code, it count all entries in a database table but i wantto count only with nid = 1
$query = 'SELECT * FROM abc WHERE nid = 
if(!empty($_POST["search"]["value"])){
$query .= 'AND (a LIKE "%'.$_POST["search"]["value"].'%" 
OR b LIKE "%'.$_POST["search"]["value"].'%" 
OR c LIKE "%'.$_POST["search"]["value"].'%") ';      
} 
if(!empty($_POST["order"])){
            $query .= 'ORDER BY '.$_POST['order']['0']['column'].' '.$_POST['order']['0']['dir'].' ';
        } else {
            $query .= 'ORDER BY id DESC ';
        }
        if($_POST["length"] != -1){
            $query .= 'LIMIT ' . $_POST['start'] . ', ' . $_POST['length'];
        }   
        $result = mysqli_query(dbConnect, $query);