A pair of functions to replace every nth occurrence of a string with another string, starting at any position in the haystack. The first works on a string and the second works on a single-level array of strings, treating it as a single string for replacement purposes (any needles split over two array elements are ignored).

Can be used for formatting dynamically-generated HTML output without touching the original generator: e.g. add a newLine class tag to every third item in a floated list, starting with the fourth item.
 
<?php
/* String Replace at Intervals   by Glenn Herbert (gjh42)    2010-12-17
 */
 
//(basic locator by someone else - name unknown)
//strnposr() - Find the position of nth needle in haystack.
function strnposr($haystack, $needle, $occurrence, $pos = 0) {
    return ($occurrence<2)?strpos($haystack, $needle, $pos):strnposr($haystack,$needle,$occurrence-1,strpos($haystack, $needle, $pos) + 1);
}

//gjh42
//replace every nth occurrence of $needle with $repl, starting from any position
function str_replace_int($needle, $repl, $haystack, $interval, $first=1, $pos=0) {
  if ($pos >= strlen($haystack) or substr_count($haystack, $needle, $pos) < $first) return $haystack;
  $firstpos = strnposr($haystack, $needle, $first, $pos);
  $nl = strlen($needle);
  $qty = floor(substr_count($haystack, $needle, $firstpos + 1)/$interval);
  do { //in reverse order
    $nextpos = strnposr($haystack, $needle, ($qty * $interval) + 1, $firstpos); 
    $qty--;
    $haystack = substr_replace($haystack, $repl, $nextpos, $nl);
  } while ($nextpos > $firstpos);
  return $haystack;
}
  //$needle = string to find
  //$repl = string to replace needle
  //$haystack = string to do replacing in
  //$interval = number of needles in loop
  //$first=1 = first occurrence of needle to replace (defaults to first) 
  //$pos=0 = position in haystack string to start from (defaults to first) 
 
//replace every nth occurrence of $needle with $repl, starting from any position, in a single-level array
function arr_replace_int($needle, $repl, $arr, $interval, $first=1, $pos=0, $glue='|+|') {
  if (!is_array($arr))  return $arr;
  foreach($arr as $key=>$value){
    if (is_array($arr[$key])) return $arr;
  }
  $haystack = implode($glue, $arr);
  $haystack = str_replace_int($needle, $repl, $haystack, $interval, $first, $pos);
  $tarr = explode($glue, $haystack);
  $i = 0;
  foreach($arr as $key=>$value){
    $arr[$key] = $tarr[$i];
    $i++;
  }
  return $arr;
}
?>
If $arr is not an array, or a multilevel array, it is returned unchanged.