<?php
function unique_chars(string $input, bool $multibyte = true): int {
    $chars = [];
    $strlen = $multibyte ? 'mb_strlen' : 'strlen';
    $substr = $multibyte ? 'mb_substr' : 'substr';
    $length = $strlen($input);

    for($i = 0; $i < $length; $i++) {
        $current = $substr($input, $i, 1);

        if(!in_array($current, $chars, true)) {
            $chars[] = $current;
        }
    }

    return count($chars);
}