code: public function getRealEncoding($encoding)
      {
          if (static::isEncodingSupported($encoding) === false) {
              throw new \Exception('Encoding is not supported: "' . $encoding . '"');
          }
  
          return static::supportedEncodings()[strtolower($encoding)];
      }
docs: Return the real encoding of the given alias
  
  @param  string $encoding
  @return string