code: private function resolveValidationRules(
          ServerConfig $config,
          OperationParams $params,
          DocumentNode $doc,
          $operationType
      ) {
          // Allow customizing validation rules per operation:
          $validationRules = $config->getValidationRules();
  
          if (is_callable($validationRules)) {
              $validationRules = $validationRules($params, $doc, $operationType);
  
              if (! is_array($validationRules)) {
                  throw new InvariantViolation(sprintf(
                      'Expecting validation rules to be array or callable returning array, but got: %s',
                      Utils::printSafe($validationRules)
                  ));
              }
          }
  
          return $validationRules;
      }
docs: @param string $operationType
  
  @return mixed[]|null