Demonstrating every 3rd post.

fiddle_476396.php

<?php
	$array = array(
		'red',
		'orange',
		'blue',
		'green',
		'yellow',
		'purple',
		'white',
		'black',
	);
echo '<ul>';
$count = 0;
while (list($key, $color) = each ($array)):
	$count++;
echo '<li>'. $color . '</li>';

$skip = 2; //Set this line to display something every X item.
if($count %$skip == 0){
	echo '<li>Every ' .$skip. ' colors, this line executes.</li>';
}
endwhile;
echo '</ul>'
?>