public static function open($tag, array $options = [], $self = false)
    {
        $attributes = self::attributes($options);

        //add the opened tag to $opened array
        if (!$self || is_null($self)) {
            self::$opened[] = $tag;
            //
            return '<'.$tag.$attributes.'>';
        } else {
            return '<'.$tag.$attributes.' />';
        }
    }