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

Going off of @Powerlord's answer,

"There is one catch: 0 % 3 is equal to 0. This could result in unexpected results if your counter starts at 0."

You can still start your counter at 0 (arrays, querys), but offset it

if (($counter + 1) % 3 == 0) {
  echo 'image file';
}

Answer 2

You can also do it without modulus. Just reset your counter when it matches.

if($counter == 2) { // matches every 3 iterations
   echo 'image-file';
   $counter = 0; 
}