php
class User extends ActiveRecord implements IdentityInterface
{
    // Regular boring things ...
}

php
class FuckingUser extends User implements FuckingIdentityInterface
{
    public function doAmazingThings()
    {
         return Yii2::getDevelopers()->makeThemJealous();
    }
}

php
interface FuckingIdentityInterface extends IdentityInterface
{
    public function doAmazingThings();
}

use yii\db\ActiveRecord;
use yii\web\IdentityInterface;

interface Test {
  public function someFuckingMethodYiiDoesntKnow(): string
}

class User extends ActiveRecord implements IdentityInterface, Test
{
    public function someFuckingMethodYiiDoesntKnow()
    {
        return 'awesome stuff';
    }
}
/**
* @var $user app\models\User
*/

$user = Yii::$app->user->identity;

/**
* @var $user app\models\User
*/
$user = Yii::$app->user->identity;