public function withProxy($proxy, $port = '', $type = '', $username = '', $password = '')
    {
        $this->withOption( 'PROXY', $proxy );

        if( !empty($port) ) {
            $this->withOption( 'PROXYPORT', $port );
        }

        if( !empty($type) ) {
            $this->withOption( 'PROXYTYPE', $type );
        }

        if( !empty($username) && !empty($password) ) {
            $this->withOption( 'PROXYUSERPWD', $username .':'. $password );
        }

        return $this;
    }