code: private function loadPersistedQuery(ServerConfig $config, OperationParams $operationParams)
      {
          // Load query if we got persisted query id:
          $loader = $config->getPersistentQueryLoader();
  
          if (! $loader) {
              throw new RequestError('Persisted queries are not supported by this server');
          }
  
          $source = $loader($operationParams->queryId, $operationParams);
  
          if (! is_string($source) && ! $source instanceof DocumentNode) {
              throw new InvariantViolation(sprintf(
                  'Persistent query loader must return query string or instance of %s but got: %s',
                  DocumentNode::class,
                  Utils::printSafe($source)
              ));
          }
  
          return $source;
      }
docs: @return mixed
  
  @throws RequestError