I would look in the code for what operation was being performed, and see if I could figure out whether the code or the comment is correct.

From a form point of view, I would also use the postfix $i++ or $i-- because of my C background. Also, I only ever use single letter variables as loop variables -- there's too much opportunity for me to forget why I called a variable 'i'.

No comment of mine would ever duplicate just what the code is doing, without an explanation of what was happening.