private function convertFunction($token, $num_args)
    {
        $args = $this->functions[$token][1];

        // Fixed number of args eg. TIME($i, $j, $k).
        if ($args >= 0) {
            return pack('Cv', $this->ptg['ptgFuncV'], $this->functions[$token][0]);
        }
        // Variable number of args eg. SUM($i, $j, $k, ..).
        if ($args == -1) {
            return pack('CCv', $this->ptg['ptgFuncVarV'], $num_args, $this->functions[$token][0]);
        }
    }