[PHP] IBAN check digits calculator (WITHOUT bcmath EXTENSION)

iban_check_digits_calculator.php

<?php
/**
 * Calculate IBAN check digits (WITHOUT bcmath EXTENSION).
 *
 * Inspired by https://github.com/arhs/iban.js/blob/master/iban.js
 *
 * @param string $countryCode ISO 3166 2-letter country code, e.g. "FR"
 * @param string $bban IBAN without country code and without check digits, e.g. "20041010050500013M02606"
 *
 * @return string e.g. "14"
 */
function calculateCheckDigits($countryCode, $bban)
{
    $temp = $bban . $countryCode . '00';
    
    $temp = strtoupper($temp);
    $temp = str_split($temp);
    $temp = array_map(function($n) {
        $code = ord($n);
        if ($code >= ord('A') && $code <= ord('Z')) {
            return $code - ord('A') + 10;
        } else {
            return $n;
        }
    }, $temp);
    $temp = implode('', $temp);
    
    $remainder = $temp;
    $block     = null;
    while (strlen($remainder) > 2) {
        $block     = substr($remainder, 0, 9);
        $remainder = (intval($block, 10) % 97) . substr($remainder, strlen($block));
    }
    $checkDigits = intval($remainder, 10) % 97;
    $checkDigits = 98 - $checkDigits;
    
    $checkDigitsAsAtring = sprintf('%02d', $checkDigits);
    
    return $checkDigitsAsAtring;
}