Increment by more than 1 in FOR loop

I’m sure this is pretty simple to do, but never having needed to do it before I’m not quite sure how to do it.
I have a FOR loop, but I want the number to increase by 15 at each iteration rather than just by one. How would I modify the $num++ to make it jump up 15 each time?

for($i = 0; $i < 100; $i += 15) {}

Ah, see? I knew it was something simple. Thank you very much.