Aerys\StandardResponse::setHeader PHP Method

    public function setHeader(string $field, string $value) : Response
    {
        if ($this->state & self::STARTED) {
            throw new \LogicException("Cannot set header; output already started");
        }
        assert($this->isValidHeaderField($field), "Invalid header field: {$field}");
        assert($this->isValidHeaderValue($value), "Invalid header value: {$value}");
        $this->headers[strtolower($field)] = [$value];
        return $this;
    }