Number to Roman digits and vice-versa converter

romanNumber.php

<?php
/**
    * Converts $n from Arabic digits to Roman digits.
    * This is accurate from 1 to 4999 and only for integers, since the
    * extensions to write the numbers greater than 4999 and the fractions are
    * not implemented.
    * Note that there is no way to write negative number ans zero, as they were
    * not used by Roman. Plus, numbers are truncated to integers, so that 4.2
    * will be converted to IV.
    *
    * @param integer $n
    * @return string
    * @throws Exception If the number cannot be converted.
**/
function arabic2roman($n) {
    $n = (int) $n;

    if($n < 1 || $n > 4999)
        throw new Exception("Cannot convert $n to roman.");

    $roman = '';

    while($n >= 1000) {
        $roman .= 'M';
        $n -= 1000;
    }

    while($n >= 100) {
        $roman .= 'C';
        $n -= 100;
    }

    $roman = str_replace('CCCCCCCCC', 'CM', $roman);
    $roman = str_replace('CCCCC', 'D', $roman);
    $roman = str_replace('CCCC', 'CD', $roman);

    while($n >= 10) {
        $roman .= 'X';
        $n -= 10;
    }

    $roman = str_replace('XXXXXXXXX', 'XC', $roman);
    $roman = str_replace('XXXXX', 'L', $roman);
    $roman = str_replace('XXXX', 'XL', $roman);

    while($n > 0) {
        $roman .= 'I';
        $n -= 1;
    }

    $roman = str_replace('IIIIIIIII', 'IX', $roman);
    $roman = str_replace('IIIII', 'V', $roman);
    $roman = str_replace('IIII', 'IV', $roman);

    return $roman;
}

/**
    * Converts $n from Roman digits to a number.
    *
    * @param string $n
    * @param int The number which corresponds to $n
    * @throws Exception If the number cannot be converted (especially because
    * of a parse error)
**/
function roman2arabic($n) {
    $arabic = 0;
    $pos = 0;
   
    $n = strtoupper($n);

    for($i = 0; $i < strlen($n); $i++) {
        $digit = $n[$i];
        $nextdigit = $n[$i+1];

        switch($digit) {
            case 'I':
                if($nextdigit == 'X') {
                    $arabic += 9;
                    $i++;
                    continue;
                }
                elseif($nextdigit == 'V') {
                    $arabic += 4;
                    $i++;
                    continue;
                }
                $arabic += 1;
                break;
            case 'V':
                $arabic += 5;
                break;
            case 'X':
                if($nextdigit == 'L') {
                    $arabic += 40;
                    $i++;
                    continue;
                }
                if($nextdigit == 'C') {
                    $arabic += 90;
                    $i++;
                    continue;
                }
                $arabic += 10;
                break;
            case 'L':
                $arabic += 50;
                break;
            case 'C':
                if($nextdigit == 'D') {
                    $arabic += 400;
                    $i++;
                    continue;
                }
                if($nextdigit == 'M') {
                    $arabic += 900;
                    $i++;
                    continue;
                }
                $arabic += 100;
                break;
            case 'D':
                $arabic += 500;
                break;
            case 'M':
                $arabic += 1000;
                break;
            default:
                throw new Exception("Parse error: $digit is not a valid Roman digit.");
        }
    }

    return $arabic;
}

header('Content-Type: text/plain');

for($i=1;$i<5000;$i++)
    echo arabic2roman($i) . ' <=> ' . roman2arabic(arabic2roman($i)) . "\n";