How to increment a string with a custom character set - php

Question

Given a custom alphabet like ['f', 'h', 'z', '#', 's']
I'd like to take a string written in this "alphabet" like
ffff#zz and increment it
So for example if the string was sss After incrementing it will look like hfff the same way that if you increment 999 you get 1000
My current attempt is here: https://3v4l.org/FjAsd
Given:
$characters = ['a', 'b', 'c'];
$string = 'cccc';
my code can do:
baaaa
but if give it
$characters = ['a', 'b', 'c'];
$string = 'aaa';
it will return
b
When I expected aab

Answer 1

The way this works is by processing the string from the end, each time you look at a character you check it's position in the array (I use a flipped array as it's more efficient than using array_search() each time). Then if the character is at the end of the array, then set it to the 0th element of the alphabet and increment the next digit to the left. If there is another letter from the alphabet to increment the current value, then just replace it and stop the loop.
The last bit is that if you have processed every character and the loop was still going, then there is a carry - so add the 0th digit to the start.
$characters = ['a', 'b', 'c'];
$string = 'cccc';
$index = array_flip($characters);
$alphabetCount = count($index)-1;
for ( $i = strlen($string)-1; $i >= 0; $i--) {
$current = $index[$string[$i]]+1;
// Carry
if ( $current > $alphabetCount ) {
$string[$i] = $characters[0];
}
else {
// update and exit
$string[$i] = $characters[$current];
break;
}
}
// As reached end of loop - carry
if ( $i == -1 ) {
$string = $characters[0].$string;
}
echo $string;
gives
aaaaa
with
$characters = ['f', 'h', 'z', '#', 's'];
$string = 'ffff#zz';
you get
ffff#z#

Answer 2

I ended up with something like this:
$string = 'ccc';
$alphabet = ['a', 'b', 'c'];
$numbers = array_keys($alphabet);
$numeric = str_replace($alphabet, $numbers, $string);
$base = count($alphabet) + 1;
$decimal = base_convert($numeric, $base, 10);
$string = base_convert(++$decimal, 10, $base);
strlen($decimal) !== strlen($string)
and $string = str_replace('0', '1', $string);
echo str_replace($numbers, $alphabet, $string);
This one has the advantage of supporting multi byte characters