Grpc\BaseStub::_validate_and_normalize_metadata PHP Method

    private function _validate_and_normalize_metadata($metadata)
    {
        $metadata_copy = [];
        foreach ($metadata as $key => $value) {
            if (!preg_match('/^[A-Za-z\\d_-]+$/', $key)) {
                throw new \InvalidArgumentException('Metadata keys must be nonempty strings containing only ' . 'alphanumeric characters, hyphens and underscores');
            }
            $metadata_copy[strtolower($key)] = $value;
        }
        return $metadata_copy;
    }