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()
        );
    }
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()
        );
    }
@param string $metadata

@return array [<placeholder> => <metadata value>]