url = $url; $this->label = $label; } public function __toString() { // There is no $this->data here return md5(serialize($this)); } public function get_url() { if ($this->url !== null) { return $this->url; } else { return null; } } public function get_attribution() { if ($this->label !== null) { return $this->label; } else { return null; } } }