wp_cache_add() │ WP 2.0.0

To demonstrate how this funciton works.

Let's take the code of function get_the_terms(). The function saves the data into the cache when called and takes the data from the cache when called again and does not make a repeated query to the database.

// I deleted some lines for clarity
function _get_the_terms( $post, $taxonomy ) {

	//try to get the cache
	$terms = get_object_term_cache( $post->ID, $taxonomy );

	if( false === $terms ){

		// no cache, gets terms
		$terms = wp_get_object_terms( $post->ID, $taxonomy );

		if ( ! is_wp_error( $terms ) ) {

			// add data to the cache
			$term_ids = wp_list_pluck( $terms, 'term_id' );
			wp_cache_add( $post->ID, $term_ids, $taxonomy . '_relationships' );
		}
	}

	return $terms;
}

This example is useless if you don't use object caching plugins. But if you do, see how much code you don't have to run thanks to the cache:

<?php
function wp_widget_recent_entries( $args ) {

	$cache_key = 'widget_recent_entries';
	$cahe_on = true;

	if( $cahe_on ){

		$output = wp_cache_get( $cache_key, 'widget' );

		if( $output ){
			echo $output;
			return;
		}

		ob_start();
	}

	extract( $args );

	$options = get_option( 'widget_recent_entries' );
	$title = empty( $options['title'] ) ? __( 'Recent Posts' ) : apply_filters( 'widget_title', $options['title'] );

	$number = (int) $options['number'];

	if( ! $number ){
		$number = 10;
	}
	elseif( $number < 1 ){
		$number = 1;
	}
	elseif( $number > 15 ){
		$number = 15;
	}

	$r = new WP_Query( [
		'posts_per_page'   => $number,
		'nopaging'         => 0,
		'post_status'      => 'publish',
		'caller_get_posts' => 1,
	] );

	if( $r->have_posts() ) :

		echo $before_widget;
		if( ! empty( $title ) ){
			echo $before_title . $title . $after_title;
		}
		?>
		<ul>
			<?php while( $r->have_posts() ) : $r->the_post(); ?>
				<li><a href="<?php the_permalink() ?>"><?= esc_html( get_the_title() ?: get_the_ID() ) ?></a></li>
			<?php endwhile; ?>
		</ul>
		<?php
		echo $after_widget;
		wp_reset_query();  // Restore global post data

	endif;

	if( $cahe_on ){
		wp_cache_add( $cache_key, ob_get_flush(), 'widget' );
	}

}
?>

You can get these statistics at the end of the PHP script (at the end of the page):

global $wp_object_cache;
$wp_object_cache->stats();

// display information about the cache