Determine a systematic element name

SystematicElementName.php

<?php

/**
 * Represents a systematic element name
 */
class SystematicElementName {
    /**
     * Gets the root matching the digit
     *
     * @param int $digit The digit
     * @param mixed $nextDigit To respect elision rules, the next digit (as int) or null if this is the last digit (facultative)
     * @return string The root
     */
    static public function getDigitRoot ($digit, $nextDigit = false) {
        switch ((int)$digit) {
            case 0: return "nil"; break;
            case 1: return "un"; break;
            case 2: return ($nextDigit === null) ? "b" : "bi"; break;
            case 3: return ($nextDigit === null) ? "tr" : "tri"; break;
            case 4: return "quad"; break;
            case 5: return "pent"; break;
            case 6: return "hex"; break;
            case 7: return "sept"; break;
            case 8: return "oct"; break;
            case 9: return ($nextDigit === 0) ? "en" : "enn"; break;
            default: throw new InvalidArgumentException("getDigitRoot: a digit were expected, got '$digit' instead.");
        }
        return $digit;
    }

    /**
     * Gets the systematic name of the element having the specified number
     *
     * @param int $number the number of the element
     * @return string the systematic name of the element
     */
    static public function getName ($number) {
        $name = [];

        $digits = str_split($number);
        $n = count($digits);
        for ($i = 0 ; $i < $n ; $i++) {
            $nextDigit = ($i < $n - 1) ? (int)$digits[$i + 1] : null;
            $name[] = self::getDigitRoot($digits[$i], $nextDigit);
        }

        $name[] = 'ium';

        return join($name);
    }
}

L11 should be @param int $lastDigit mixed (int|null)

Done.