Caesar Cipher Encode Decode (PHP)

CaesarCipher.php

<?php
function CaesarCipherDecode($string, $casas = 9) {
  $abc = range('a', 'z');
  $decifrado = '';
  foreach (str_split(strtolower($string)) as $letter) {
      if (ctype_alpha($letter)) {
          foreach ($abc as $key => $alpha) {
              if ($letter === $alpha) {
                  if (($key - $casas) < 0) {
                      $decifrado .= $abc[(($key - $casas) + count($abc))];
                  } else {
                      $decifrado .= $abc[($key - $casas)];
                  }
              }
          }
      } else {
          $decifrado .= $letter;
      }
  }
  return $decifrado;
}

function CaesarCipherEncode($string, $casas = 9) {
  $abc = range('a', 'z');
  $cifrado = '';
  foreach (str_split(strtolower($string)) as $letter) {
      if (ctype_alpha($letter)) {
          foreach ($abc as $key => $alpha) {
              if ($letter === $alpha) {
                  if (($key + $casas) >= count($abc)) {
                      $cifrado .= $abc[(($key + $casas) - count($abc))];
                  } else {
                      $cifrado .= $abc[($key + $casas)];
                  }
              }
          }
      } else {
          $cifrado .= $letter;
      }
  }
  return $cifrado;
}

$string = 'Jessica, eu te amo!';

echo CaesarCipherEncode($string) . '<br />' . CaesarCipherDecode(CaesarCipherEncode($string));