code: private function getMetadataMap($metadata)
      {
          return array_reduce(
              $this->placeholders,
              function (array $acc, $definition) use ($metadata) {
                  return array_merge($acc, array($definition['placeholder'] => $definition[$metadata]));
              },
              array()
          );
      }
docs: @param string $metadata
  
  @return array [<placeholder> => <metadata value>]