code: public function parse($content = '')
      {
          if (empty($content)) {
              return '';
          }
  
          for ($i = 0, $count = count($this->ubb); $i < $count; $i++) {
              if (count($this->ubb[$i]) == 4) {
                  //解析闭合标签
                  $content = $this->closeTag($content, $this->ubb[$i]);
              } else {
                  $content = $this->onceTag($content, $this->ubb[$i]);
              }
          }
  
          return nl2br($content);
      }
docs: 解析UBB代码为HTML
  @param  string $content 要解析的UBB代码
  @return string          解析后的HTML代码