PHP: How do you determine every Nth iteration of a loop?

Question

I wanted to echo an image every after 3 post via XML here is my code :

Answer 1

It will not work for first position so better solution is :

if ($counter != 0 && $counter % 3 == 0) {
   echo 'image file';
}

Check it by yourself. I have tested it for adding class for every 4th element.