Where to put plain object classes?

In your opinion, what's the correct place to put a plain object class file? The class I need is something similar to

PHP Code:

class PlainObject {

    private $id;

    public function getID() {
        return $this->id;
    }

} 


and I want to use this class wherever I need it (a controller or a model or a helper or a library, etc). Any suggestion?