system/HTTP/Response.php
public function getReason(): string
	{
		if (empty($this->reason))
		{
			return ! empty($this->statusCode) ? static::$statusCodes[$this->statusCode] : '';
		}

		return $this->reason;
	}
public function getReason(): string
	{
		if (empty($this->reason))
		{
			return ! empty($this->statusCode) ? static::$statusCodes[$this->statusCode] : '';
		}

		return $this->reason;
	}
Gets the response response phrase associated with the status code.

@see http://tools.ietf.org/html/rfc7231#section-6
@see http://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml

@return string