Thank you for your comment.

I put this solution to solve the kata only and didn't think about best solutions.

Your piece of code is really clear and I already put it in my toolbox. (y)

I'll try to use your solution in the next time.

You can use less commands when making sure $char is not more than 255. Just use $char = ++$char % 256; -- this way it is shorter, cleaner and is only one command instead of if/else. Just an advice.