PHP - problema al buscar datos

<?php

include "conexion.php";

    if ($_SERVER['REQUEST_METHOD'] == 'GET'){

        $mysqli= new mysqli('localhost', 'root', '', 'oficina');

        $ID = isset($_GET['ID']) ? $_GET['ID'] : '';

        $query = "Select * from casos Where ID = '$ID'";

        $result = $mysqli->query($query);

            if ($mysqli -> affected_rows > 0){

                while ($row = $result->fetch_assoc()){

                $array = $row;

                }

                echo json_encode($array);

            } else {

                echo "No se encontraron datos";

            }

$result ->close();

$mysqli ->close();

    }

?>