code: public function usort($cmp_function)
      {
          $tmp = $this->getArrayCopy();
          $ret = usort($tmp, $cmp_function);
  
          $tmp = new self($tmp);
          $this->exchangeArray($tmp->getArrayCopy());
  
          return $ret;
      }
docs: Adds usort as an instance method
  
  @param callback $cmp_function Function to sort by
  @return boolean