<?php
$path = parse_url($_SERVER['REQUEST_URI'], PHP_URL_PATH);

header('Content-Type: application/json');

if ($path === '/users/42') {
    echo json_encode([
        'id' => 42,
        'name' => 'Ada'
    ]);
    return;
}

http_response_code(404);
echo json_encode([
    'error' => 'user not found'
]);