declare property as object?

From the PHP manual on class properties (emphasis mine):

Class member variables are called "properties". You may also see them referred to using other terms such as "attributes" or "fields", but for the purposes of this reference we will use "properties". They are defined by using one of the keywords public, protected, or private, followed by a normal variable declaration. This declaration may include an initialization, but this initialization must be a constant value --that is, it must be able to be evaluated at compile time and must not depend on run-time information in order to be evaluated.

Either create it inside the constructor (composition)

class Foo
{
    protected $bar;
    public function __construct()
    {
        $this->bar = new Bar;   
    }
}

or inject it in the constructor (aggregation)

class Foo
{
    protected $bar;
    public function __construct(Bar $bar)
    {
        $this->bar = $bar;   
    }
}

or use setter injection.

class Foo
{
    protected $bar;
    public function setBar(Bar $bar)
    {
        $this->bar = $bar
    }
}

You want to favor aggregation over composition.

You can create a new class in constructer area then set it as new object.

class CampaignGroupsProperty
{
    public $id;
    public $name;
}



class GetCampaignGroupsResponse
{
    public $result;
    public $resultCode;

    public $campaignGroups;

    public function __construct()
    {
        $this->campaignGroups = new CampaignGroupsProperty();
    }
}

Then you can call it like

$response = new GetCampaignGroupsResponse();
$response->campaignGroups->id = 'whatever';
$response->result=true;

If you are just looking to instantiate to a generic class you can do:

$objectname = new stdClass;

I don't believe you can do this in the declaration of a property so you'd have to just declare $objectname and in the constructor set it to new stdClass.