Zend\Diactoros\AbstractSerializer::serializeHeaders PHP Method

    protected static function serializeHeaders(array $headers)
    {
        $lines = [];
        foreach ($headers as $header => $values) {
            $normalized = self::filterHeader($header);
            foreach ($values as $value) {
                $lines[] = sprintf('%s: %s', $normalized, $value);
            }
        }
        return implode("\r\n", $lines);
    }