XML 解析错误:找不到根元素位置

here is my login.php

<?php
header('Content-type: application/json');

$servername = "localhost";
$username = "root";
$password = "";
$dbname = "jammer";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    header('HTTP/1.1 500 Bad connection to Database');
    die("The server is down, we couldn't establish the DB connection");
}
else {
    $conn ->set_charset('utf8_general_ci');
    $userName = $_POST['username'];
    $userPassword = $_POST['userPassword'];

    $sql = "SELECT username, firstName, lastName FROM users WHERE username = '$userName' AND password = '$userPassword'";
    $result = $conn->query($sql);

    if ($result->num_rows > 0) {
        while($row = $result->fetch_assoc()) {
            $response = array('firstName' => $row['firstNameName'], 'lastName' => $row['lastName']);
        }
        echo json_encode($response);
    }
    else {
        header('HTTP/1.1 406 User not found');
        die("Wrong credentials provided!");
    }
}
$conn->close();
?>

我已经对 xml 解析错误进行了一些研究，但我仍然无法使我的项目正常工作，我尝试过使用 Google Chrome 和 Firefox