PHPUnit: mock non-existing classes

Is it possible to create a mock of non-existing class in PHPUnit?

Let's assume I have some class that creates instance of another class, for example:

class TaskRunner
{
    public function runTasks()
    {
        // Run through some loop to get tasks and store each in $taskName
        // Get task instance by given task name 
        $task = $this->getTaskInstance($taskName);
        if ($task instanceof AbstractTask) {
            $task->run();
        }
    }

    protected function getTaskInstance($taskName)
    {
        // Just an example
        return new $taskName();
    }
}

I would like to run unit test for runTasks method to check if created task instace extends some abstract class.

Is there any possibility to NOT to create sample class in a filesystem to check the inheritance constraint?

Thanks for all!