yeah, it's called returnary

Why do you need to make it as short as possible?

My brain needed time to interpret the statement. Indeed confusing

Thanks for noticing my do-while solution.
About the "$valid" variables maybe you are right, but I was having fun :)

This is the whole trick, to make the code as short as possible. But I agree with you, this is really very difficult to read.

True, and its more readable too.

That was the one of the tricky parts of this one!

I have just made a more concise solution ;) https://www.codewars.com/kata/reviews/57cb045535ef62f92d0000f1/groups/610d7e5b96e25100013cae0d

Right, as I said, PHP is loosely typed. In any case, 1 and 0 are perfectly valid ways to represent truth.

0.1 + 0.2 in javascript... is just a representation of how float values are imprecise. It's impossible to represent all decimals in binary. It's common in every language. PHP has these too. Try echo 0.1 + 0.2 == .3 ? "Equal" : "Not Equal";

Thanks for your comment.

Please use a spoiler flag when divulgating elements of a solution. This is visible by anyone from the Dashboard.

0 and 1 ARE boolean values. PHP makes it even more confusing because it's loosely typed. Try doing this sometime: echo true+true+true;

And that's correct since following the rules of this kata, if the window==0 bouncing never ends.