Increment on non-numeric string is deprecated, use str_increment() instead

[KarelWintersky]

https://github.com/nikic/FastRoute/blob/master/src/DataGenerator/MarkBased.php

/** @inheritDoc */
    protected function processChunk(array $regexToRoutesMap): array
    {
        $routeMap = [];
        $regexes = [];
        $markName = 'a';

        foreach ($regexToRoutesMap as $regex => $route) {
            $regexes[] = $regex . '(*MARK:' . $markName . ')';
            $routeMap[$markName] = [$route->handler, $route->variables, $route->extraParameters];

            ++$markName; // HERE
        }

        $regex = '~^(?|' . implode('|', $regexes) . ')$~';

        return ['regex' => $regex, 'routeMap' => $routeMap];
    }

See https://www.php.net/manual/en/migration83.deprecated.php

PHP 8.5