public static function digit_to_word($num)
    {
        $numbers = array(_t('MathSpamProtection.ZERO', 'zero'),
            _t('MathSpamProtection.ONE', 'one'),
            _t('MathSpamProtection.TWO', 'two'),
            _t('MathSpamProtection.THREE', 'three'),
            _t('MathSpamProtection.FOUR', 'four'),
            _t('MathSpamProtection.FIVE', 'five'),
            _t('MathSpamProtection.SIX', 'six'),
            _t('MathSpamProtection.SEVEN', 'seven'),
            _t('MathSpamProtection.EIGHT', 'eight'),
            _t('MathSpamProtection.NINE', 'nine'),
            _t('MathSpamProtection.TEN', 'ten'),
            _t('MathSpamProtection.ELEVEN', 'eleven'),
            _t('MathSpamProtection.TWELVE', 'twelve'),
            _t('MathSpamProtection.THIRTEEN', 'thirteen'),
            _t('MathSpamProtection.FOURTEEN', 'fourteen'),
            _t('MathSpamProtection.FIFTEEN', 'fifteen'),
            _t('MathSpamProtection.SIXTEEN', 'sixteen'),
            _t('MathSpamProtection.SEVENTEEN', 'seventeen'),
            _t('MathSpamProtection.EIGHTEEN', 'eighteen'));

        if ($num < 0) {
            return "minus ".($numbers[-1*$num]);
        }

        return $numbers[$num];
    }