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

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

<?php
// URL of the XML feed.
$feed = 'test.xml';
// How many items do we want to display?
//$display = 3;
// Check our XML file exists
if(!file_exists($feed)) {
  die('The XML file could not be found!');
}
// First, open the XML file.
$xml = simplexml_load_file($feed);
// Set the counter for counting how many items we've displayed.
$counter = 0;
// Start the loop to display each item.
foreach($xml->post as $post) {
  echo ' 
  <div style="float:left; width: 180px; margin-top:20px; margin-bottom:10px;">
 image file</a> <div class="design-sample-txt">'. $post->author.'</div></div>
';

  // Increase the counter by one.
  $counter++;
  // Check to display all the items we want to.
  if($counter >= 3) {
    echo 'image file';
    }
  //if($counter == $display) {
    // Yes. End the loop.
   // break;
  //}
  // No. Continue.
}
?>

here is a sample first 3 are correct but now it doesn't loop idgc.ca/web-design-samples-testing.php

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; 
}

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.

How about: if(($counter % $display) == 0)

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';
}

every 3 posts?

if($counter % 3 == 0){
    echo IMAGE;
}

Use the modulo arithmetic operation found here in the PHP manual.

e.g.

$x = 3;

for($i=0; $i<10; $i++)
{
    if($i % $x == 0)
    {
        // display image
    }
}

For a more detailed understanding of modulus calculations, click here.

I am using this a status update to show a "+" character every 1000 iterations, and it seems to be working good.

if ($ucounter % 1000 == 0) { echo '+'; }