How can i solve it?

It’s giving me error:
Warning: Undefined array key “id” in D:xampphtdocsBlogFulladminedit-user-logic.php on line 5

Fatal error: Uncaught mysqli_sql_exception: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ‘LIMIT 1’ at line 1 in D:xampphtdocsBlogFulladminedit-user-logic.php:14 Stack trace: #0 D:xampphtdocsBlogFulladminedit-user-logic.php(14): mysqli_query(Object(mysqli), ‘UPDATE users SE…’) #1 {main} thrown in D:xampphtdocsBlogFulladminedit-user-logic.php on line 14

I tried replacing single quotes with ` , but it didn’t help

<?php 
require 'config/database.php';

if(isset($_POST['submit'])) {
    $id = filter_var($_POST['id'], FILTER_SANITIZE_NUMBER_INT);
    $firstname = filter_var($_POST['firstname'], FILTER_SANITIZE_FULL_SPECIAL_CHARS);
    $lastname = filter_var($_POST['lastname'], FILTER_SANITIZE_FULL_SPECIAL_CHARS);
    $is_admin = filter_var($_POST['userrole'], FILTER_SANITIZE_NUMBER_INT);

    if(!$firstname || !$lastname) {
        $_SESSION['edit-user'] = "Invalid form input on edit page.";
    } else {
        $query = "UPDATE users SET firstname='$firstname', lastname='$lastname', is_admin=$is_admin WHERE id=$id LIMIT 1";
        $result = mysqli_query($connection, $query);

        if(mysqli_errno($connection)) {
            $_SESSION['edit-user'] = "Failed to update user.";
        } else {
            $_SESSION['edit-user-success'] = "User $firstname $lastname updated sucessfully";

        }

    }
}

header('location: ' . ROOT_URL . 'admin/manage-users.php');
die();