All the properties of your object are private. aka... not available outside their class's scope.

Solution for PHP >= 5.4

Use the new JsonSerializable Interface to provide your own json representation to be used by json_encode

class Thing implements JsonSerializable {
    ...
    public function jsonSerialize() {
        return [
            'something' => $this->something,
            'protected_something' => $this->get_protected_something(),
            'private_something' => $this->get_private_something()
        ];
    }
    ...
}

Solution for PHP < 5.4

If you do want to serialize your private and protected object properties, you have to implement a JSON encoding function inside your Class that utilizes json_encode() on a data structure you create for this purpose.

class Thing {
    ...
    public function to_json() {
        return json_encode(array(
            'something' => $this->something,
            'protected_something' => $this->get_protected_something(),
            'private_something' => $this->get_private_something()                
        ));
    }
    ...
}

A more detailed writeup