Ad every Nth

You can use Modulus division to add in every Nth (in this case 6th) instance. So…
  if ($i == 6 || $i == 12 || $i == 18)
would be
  if ($i % 6 == 0)
One pitfall with this is that 0 % 3 = 0 so with the counter ($i) starting at 0 you could also get an ad at the beginning of the loop. Deans example adds 1 to $i before the first iteration of the loop anyway so that shouldn’t be an issue. But just in case, to stop that you can use.
  if ($i > 0 && $i % 6 == 0)