ValueObjectAllTheThings:RomanNumeralEdition!.php

<?php
final class Number // Immutable value object
{
    /**
     * @param int $arabic
     * @throws InvalidArgumentException
     */
    public function __construct($arabic) { /* ... */ }

    /**
     * @param int $arabic
     * @return Number
     * @throws InvalidArgumentException
     */
    public static function fromArabic($arabic) { /* ... */ }

    /**
     * @param string $roman
     * @return Number
     * @throws InvalidArgumentException
     */
    public static function fromRoman($roman) { /* ... */ }

    /**
     * @return int
     */
    public function asArabic() { /* ... */ }

    /**
     * @return string
     */
    public function asRoman() { /* ... */ }
}