Kelpie\Server\Headers::offsetSet PHP Метод

    public function offsetSet($key, $value)
    {
        if (!isset($this->_sent[$key]) || in_array($key, self::$ALLOWED_DUPLICATES)) {
            $this->_sent[$key] = true;
            if (!isset($value)) {
                return;
            } elseif ($value instanceof DateTime) {
                $value = $value->format(DateTime::RFC1123);
            } else {
                $value = "{$value}";
            }
            $this->_out[] = sprintf(self::$HEADER_FORMAT, $key, $value);
        }
        return $this->_sent[$key] = $value;
    }