code: public static function encode($content, $doubleEncode = true)
      {
          return htmlspecialchars($content, ENT_QUOTES | ENT_SUBSTITUTE, Yii::$app ? Yii::$app->charset : 'UTF-8', $doubleEncode);
      }
docs: Encodes special characters into HTML entities.
  The [[\yii\base\Application::charset|application charset]] will be used for encoding.
  @param string $content the content to be encoded
  @param bool $doubleEncode whether to encode HTML entities in `$content`. If false,
  HTML entities in `$content` will not be further encoded.
  @return string the encoded content
  @see decode()
  @see https://secure.php.net/manual/en/function.htmlspecialchars.php