public function protocol($protocol = null)
    {
        if ($protocol !== null) {
            $this->protocol = $protocol;
        }

        return $this->protocol;
    }
public function protocol($protocol = null)
    {
        if ($protocol !== null) {
            $this->protocol = $protocol;
        }

        return $this->protocol;
    }
public function protocol ( $ protocol = null ) { if ( $ protocol !== null ) { $ this -> protocol = $ protocol ; } return $ this -> protocol ; }
Returns response protocol and its version

@param null|string $protocol

@return string
Returns response protocol and its version