Why the answer is 0?

Question

It's a challenge question and found answer 0 but don't know how. Can anyone please explain? Check the attached Screenshot.

https://photos.app.goo.gl/oE8XPESjPi7kcP539

Answer 1

In echo statement you can not use plus sign to join two strings.

Answer 2

To concatenate we use '.' symbol'+' is for numbers. Here string as a number is zero. Adding two strings is 0 + 0=0

Answer 3

Thanks everyone I got now.