<?php

function decrement_letters($str) {
  $letters = str_split($str);
  $c = count($letters);

  if ($c === 1) {
    $letters[0] = $letters[0] === 'A' ? 'A' : chr(ord($letters[$c-1]) - 1);
  } elseif($letters[$c-1] !== 'A') {
    $letters[$c-1] = chr(ord($letters[$c-1]) - 1);
  } elseif($letters[$c-1] === 'A' && $letters[$c-2] === 'A') {
    $letters[$c-2] = 'Z';
    array_pop($letters);
  } else {
    $letters[$c-1] = 'Z';
    $letters[$c-2] = chr(ord($letters[$c-2]) - 1);;
  }

  return implode('', $letters);
}