<?php

$str = 'a';
printf("++'%s' = %s\n", $str, ++$str);

$str = 'z';
printf("++'%s' = %s\n", $str, ++$str);

$str = 'aa';
printf("++'%s' = %s\n", $str, ++$str);

$str = 'az';
printf("++'%s' = %s\n", $str, ++$str);