s_vs_sticky.php

<?php 
/* *** SITUATION ***
 * We want to retrieve 3 posts and show the sticky posts on top.
 * To make sticky posts work as expected, the 's' parameter must be not declared or should be NULL,
 * otherwise the sticky posts won't appear on top of other posts.
 */
 
 /* *** CASE 1 *** 
 * This code works as expected ('s' is not declared) and sticky posts appear on top:
 */
$params = array(
	'post_type'           => 'post',
	'posts_per_page'      => 3,
	'ignore_sticky_posts' => false,
	// 's'                => '',
);
$my_query = new WP_Query( $params );
if ( $my_query->have_posts() ) : while( $my_query->have_posts() ) : $my_query->the_post(); ?>
	<h2><?php the_title(); ?></h2>
<?php endwhile; endif; ?>
<hr>


<?php
 /* *** CASE 2 *** 
 * This code works as expected ('s' is NULL) and sticky posts appear on top:
 */
$params = array(
	'post_type'           => 'post',
	'posts_per_page'      => 3,
	'ignore_sticky_posts' => false,
	's'                   => NULL,
);
$my_query = new WP_Query( $params );
if ( $my_query->have_posts() ) : while( $my_query->have_posts() ) : $my_query->the_post(); ?>
	<h2><?php the_title(); ?></h2>
<?php endwhile; endif; ?>
<hr>


<?php
 /* *** CASE 3 *** 
 * This code fails ('s' is declared and is empty) and sticky posts behave as normal posts:
 */
$params = array(
	'post_type'           => 'post',
	'posts_per_page'      => 3,
	'ignore_sticky_posts' => false,
	's'                   => '',
);
$my_query = new WP_Query( $params );
if ( $my_query->have_posts() ) : while( $my_query->have_posts() ) : $my_query->the_post(); ?>
	<h2><?php the_title(); ?></h2>
<?php endwhile; endif;

/* SO THE QUESTION IS:
 * Why couldn't the 's' parameter be empty ('s' = '')
 * to make sticky posts work as expected?
 */