URL
    http://example.com/api/user_data/1

    JSON
    {
      "success": true,
      "data": {
        "id": 1,
        "display_name": "your_username",
        "user_registered": "2015-11-12 05:00:00",
        "first": "First",
        "last": "Last Name"
      }
    }

if ( ! class_exists( 'JSONEndpoint_UserData' ) ):
    /**
     * The code that registers the endpoint and handles the result
     */
    class JSONEndpoint_UserData {

        const ENDPOINT_NAME       = 'api/user_data'; // endpoint to capture
        const ENDPOINT_QUERY_NAME = '__api_user_data'; // turns to param

        // WordPress hooks
        public function run() {
            add_filter( 'query_vars', array( $this, 'add_query_vars' ), 0 );
            add_action( 'parse_request', array( $this, 'sniff_requests' ), 0 );
            add_action( 'init', array( $this, 'add_endpoint' ), 0 );
        }

        // Add public query vars
        public function add_query_vars( $vars ) {
            $vars[] = static::ENDPOINT_QUERY_NAME;
            $vars[] = 'id';

            return $vars;
        }

        // Add API Endpoint
        public function add_endpoint() {
            add_rewrite_rule( '^' . static::ENDPOINT_NAME . '/([^/]+)/?$', 'index.php?' . static::ENDPOINT_QUERY_NAME . '=1&id=$matches[1]', 'top' );
// --->
            flush_rewrite_rules( true ); //// <---------- REMOVE THIS WHEN DONE TESTING
// --->
        }

        // Sniff Requests
        public function sniff_requests( $wp_query ) {

            global $wp;

            if ( isset( $wp->query_vars[ static::ENDPOINT_QUERY_NAME ] ) ) {
                $this->handle_request(); // handle it
            }
        }

        // Handle Requests
        protected function handle_request() {
            global $wp;

            // we only deal with number$
            $id = is_numeric( $wp->query_vars[ 'id' ] ) ? absint( $wp->query_vars[ 'id' ] ) : false;

            if ( ! is_numeric( $id ) || ! $user = get_user_by( 'id', $id ) ) {
                wp_send_json_error( array( 'message' => 'Invalid User ID' ) );
            }

            // ALLOWING ACCESS FROM ANYWHERE --- WE MIGHT WANT TO RESTRICT THE PLACES THAT CAN USE THIS
            header( "Access-Control-Allow-Origin: *" );

            // prep the response
            $data = array(
                'id'              => $user->ID,
                'display_name'    => $user->data->display_name,
                'user_registered' => $user->data->user_registered,
                'first'           => $user->first_name,
                'last'            => $user->last_name,
            );

            // write the response
            wp_send_json_success( $data );

            die(); // just in case
        }
    }

    $ep = new JSONEndpoint_UserData();
    $ep->run();

endif; // JSONEndpoint_UserData