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);
    }
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
Callback .