SassNumber::unitString PHP Method

    public function unitString($numeratorUnits, $denominatorUnits)
    {
        foreach ($numeratorUnits as $i => $unit) {
            if (!in_array($unit, self::$validUnits)) {
                unset($numeratorUnits[$i]);
            }
        }
        foreach ($denominatorUnits as $i => $unit) {
            if (!in_array($unit, self::$validUnits)) {
                unset($denominatorUnits[$i]);
            }
        }
        return join(' * ', $numeratorUnits) . (!empty($denominatorUnits) ? ' / ' . join(' * ', $denominatorUnits) : '');
    }