// Pragmatic, yet flexible solution.

interface JsonConvertable {
  function toJson();
}

class Mammal implements JsonConvertable {
  private $id;

  function toJson() {
    return json_encode($this->getJsonState());
  }

  protected function getJsonState() {
    return get_object_vars($this);
  }
}

class Ape extends Mammal {
  private $name;

  protected function getJsonState() {
    return array_merge(parent::getJsonState(), get_object_vars($this));
  }
}

$ape = new Ape;
var_dump($ape->toJson());