src/StringConverter/Converter.php
final protected function callback(string $string, callable $callback = null): string
    {
        if (!is_callable($callback)) {
            return $string;
        }

        return call_user_func($callback, $string);
    }
final protected function callback(string $string, callable $callback = null): string
    {
        if (!is_callable($callback)) {
            return $string;
        }

        return call_user_func($callback, $string);
    }
Callback.

@param  string $string
@param  callable|null $callback
@return string