code: public function getSignableParameters()
      {
          // Grab all parameters
          $params = $this->parameters;
  
          // Remove oauth_signature if present
          // Ref: Spec: 9.1.1 ("The oauth_signature parameter MUST be excluded.")
          if (isset($params['oauth_signature'])) {
              unset($params['oauth_signature']);
          }
  
          return Util::buildHttpQuery($params);
      }
docs: The request parameters, sorted and concatenated into a normalized string.
  
  @return string