code: public function buildFormDataString($options)
      {
          $options = $this->resolve($options);
          $formOptions = [];
  
          foreach ($this->formParameters as $key => $isFormParameter) {
              if ($isFormParameter && isset($options[$key])) {
                  $formOptions[$key] = $options[$key];
              }
          }
  
          return http_build_query($formOptions);
      }
docs: Build form data string.
  
  @param $options
  
  @return string