I'd like for all of my objects to be able to return a JSON string of themselves.

So I created a base class for all of my objects to extend, with an AsJSON() method:

class BaseObject {

   public function AsJSON()
   {
      $JSON=array();
      foreach ($this as $key => $value)
      {
          if(is_null($value))
            continue;
          $JSON[$key] = $value;
      }
      return json_encode($JSON);
   }
}

And then extend my child classes from that:

class Package extends BaseObject {
   ...
}

So in my code, I expect to do this:

$Box = new Package;
$Box->SetID('123');
$Box->SetName('12x8x6');
$Box->SetBoxX('12');
$Box->SetBoxY('8');
$Box->SetBoxZ('6');
echo $Box->AsJSON();

But the JSON string it returns only contains the BaseClass's properties, not the child properties.

How do I modify my AsJSON() function so that $this refers to the child's properties, not the parent's?