Solução do Pergaminho de Fasdal

pergaminho.php

<?php

/**
 * Usage: run `php pergaminho.php` on Terminal
 * 
 * @author Cadu de Castro Alves <https://github.com/castroalves>
 */

$encrypted_message = 'VNWRWJ BQJAXW YXBBDR BJWPDN ANJU. YANLRBJVXB MNUJ YJAJ ANBBDBLRCJA X NGNALRCX MN VXACXB N RWRLRJA J MNBCADRLJX CXCJU MN OJBMJU.';
$decrypted_message = 'FASDAL DE CU EH ROLA';

echo decryptMessage($encrypted_message, 10);

echo encryptMessage($decrypted_message, 10);

/**
 * Creates an array with the respective offset alphabet
 *
 * @param int       $offset     Offset of the alphabet
 *
 * @return array
 */
function offsetAlphabet(int $offset): array
{
    $letters = range('A', 'Z');
    $total_letters = count($letters);
    $offset_alphabet = [];
    foreach ($letters as $index => $letter) {
        $offset_index = $index + $offset - 1;
        $calc_index = $total_letters - $offset_index;
        $current_index = $calc_index > 0 ? $offset_index : abs($calc_index);
        $offset_alphabet[$letters[$current_index]] = $letter;
    }
    return $offset_alphabet;
}

/**
 * Decrypt message using the given offset
 *
 * @param string    $message    Message to decrypt
 * @param int       $offset     Offset of the alphabet
 *
 * @return string
 */
function decryptMessage(string $message, int $offset = 10): string
{
    $offset_alphabet = offsetAlphabet($offset);
    return strtr(strtoupper($message), $offset_alphabet) . PHP_EOL;
}

/**
 * Encrypt message for the given offset
 *
 * @param string    $message    Message to encrypt
 * @param int       $offset     Offset of the alphabet
 *
 * @return string
 */
function encryptMessage(string $message, int $offset = 10): string
{
    $offset_alphabet = array_flip(offsetAlphabet($offset));
    return strtr(strtoupper($message), $offset_alphabet) . PHP_EOL;
}