如果我们已经使用了继承，如何实现观察？

There is a ForumThread class:

class ForumThread extends DbTable
{
    public function insert ($threadId, $comment)
    {
        SQL INSERT INTO parent::tablename VALUES $threadId, $comment
        // email sending how?
        // putting this on a "notice-wall", how?
    }
}

some other functions should be done here, e.g. email sending. I cant put it here otherwise I violate the SRP. I cant put it neither to controller, as I want to insert post elsewhere too. Im planning to implement the Observed pattern, but I cant extend from two classes.

With the observer pattern you would have to dispatch a notification from this method in order to execute the relevant observers code.

You would execute something like this from within the insert method:

$this->notify('table_insertion', $data);

Then somewhere else before the notify line is executed, the event must be registered like so:

static::$observers['table_insertion'][] = array('class_to_call' => 'method_in_class_to_call');

The notify method would be something like:

public function notify($event, $data) {
  foreach(static::$observer[$event] as $class => $method) {
    new $class->$method($data);
  }
}

Hope this makes sense.