I have this code block:

class ExampleService extends BaseService
{
    protected $handler;

    public const TASK_ONE = 'task_one';
    public const TASK_TWO = 'task_two';

    public function __construct()
    {
        $model = new GenericModel();

        $model->setTaskList([
            (new GenericTask(self::TASK_ONE))
                ->setPriority(0)
                ->setTaskType(GenericTask::TYPE_IMPORT)
                ->setCallback([$this->handler, 'runTaskOne']),

            (new GenericTask(self::TASK_TWO))
                ->setPriority(100)
                ->setTaskType(GenericTask::TYPE_EXPORT)
                ->setCallback([$this->handler, 'runTaskTwo']),
        ]);

        parent::__construct($model);
    }

    public static function getInstance()
    {
        static $instance;

        if ($instance === null) {
            $instance = new self();
        }

        return $instance;
    }
}

my issue is that when i reload the page i get a fatal error on this line

(new GenericTask(self::TASK_ONE))

which says something like "GenericTask is not callable". After hitting my head in the wall a few times and after several debugs and tries i noticed that if i change that self::TASK_ONE to something like static::TASK_ONE or even $this::TASK_ONE then everything works fine

What are the forces at play here? why only self:: fails? Can somebody explain ?