Initialising and using Transients in WordPress

functions.php

<?php 
// Check for transient. If none, then execute WP_Query
if ( false === ( $featured = get_transient( 'foo_featured_posts' ) ) ) {
	$featured = new WP_Query(
		array(
			'category' => 'featured',
			'posts_per_page' => 5
		)
	);

	// Put the results in a transient. Expire after 12 hours.
	set_transient( 'foo_featured_posts', $featured, 12 * HOUR_IN_SECONDS );
}
?>
 
// Run the loop as normal

<?php if ( $featured->have_posts() ) : ?>

	<?php while ( $featured->have_posts() ) : $featured->the_post(); ?>
		// featured posts found, do stuff
	<?php endwhile; ?>

<?php else: ?>
	// no featured posts found
<?php endif; ?>

<?php wp_reset_postdata(); ?>