public function execute() {
    // Make sure this runs only once.
    if($this->priorities) {
      // Sort the functions by their priority.
      if(ksort($this->callbacks)) {
        foreach($this->callbacks as $callbacks) {
          foreach($callbacks as $callback) {
            call_user_func($callback);
          }
        }
      }

      // Destroy instance.
      $this->priorities = null;
      $this->callbacks = null;
    }
  }