code: protected function doFunction_setExpressCheckout(array $fields)
      {
          $request = new FormRequest;
          $request->setFields($fields);
  
          if (!isset($fields['return_url'])) {
              if (false == $this->options['return_url']) {
                  throw new RuntimeException('The return_url must be set either to FormRequest or to options.');
              }
  
              $request->setField('return_url', $this->options['return_url']);
          }
  
          if (!isset($fields['cancel_url'])) {
              if (false == $this->options['cancel_url']) {
                  throw new RuntimeException('The cancel_url must be set either to FormRequest or to options.');
              }
  
              $request->setField('cancel_url', $this->options['cancel_url']);
          }
  
          $request->setField('function', 'SetExpressCheckout');
  
          $this->addVersionField($request);
          $this->addAuthorizeFields($request);
  
          return $this->doRequest($request);
      }
docs: Require:
  
  @param array $fields
  
  @return array