/**
* function get_id_by_url()
* 根据url获取对应的id号
* @param string $url
*/
public function get_id_by_url($url = FALSE) {
$url = $url ? $url : $this->url;
$result = $this->db->select ( 'id' )->from ( 'url_info' )->where ( 'url', $url )->get ()->row ( 0 );
return $result->id;
}