code: public static function a($text, $url = null, $options = [])
      {
          if ($url !== null) {
              $options['href'] = Url::to($url);
          }
  
          return static::tag('a', $text, $options);
      }
docs: Generates a hyperlink tag.
  @param string $text link body. It will NOT be HTML-encoded. Therefore you can pass in HTML code
  such as an image tag. If this is coming from end users, you should consider [[encode()]]
  it to prevent XSS attacks.
  @param array|string|null $url the URL for the hyperlink tag. This parameter will be processed by [[Url::to()]]
  and will be used for the "href" attribute of the tag. If this parameter is null, the "href" attribute
  will not be generated.
  
  If you want to use an absolute url you can call [[Url::to()]] yourself, before passing the URL to this method,
  like this:
  
  ```php
  Html::a('link text', Url::to($url, true))
  ```
  
  @param array $options the tag options in terms of name-value pairs. These will be rendered as
  the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
  If a value is null, the corresponding attribute will not be rendered.
  See [[renderTagAttributes()]] for details on how attributes are being rendered.
  @return string the generated hyperlink
  @see \yii\helpers\Url::to()