What happens in my confuse coding i don't understand?

Question

php-increment & decrement topic
https://code.sololearn.com/w6GFWeFUrE5L/?ref=app

Answer 1

that's because of ASCII values , when you increment an alphabet , it increments its ASCII value and the converts it to character , thus you get the output aa.

Answer 2

https://www.sololearn.com/discuss/489143/?ref=app

Answer 3

I understand because of increment of after z comes aa similarly if zz comes aaa then comes.

Answer 4

increment means add 1 value,
decrement means substract 1 value