WordPress: Similar posts by matching tags

similar_posts.php

<?php 
/**
 * Return posts with maximum age of one year.
 *
 * Usage:
 *   add_filter( 'posts_where', 'wp_query_filter_max_one_year_old' );
 *   $my_query = new WP_Query( array( ... ) );
 *   remove_filter( 'posts_where', 'wp_query_filter_max_one_year_old' );
 * 
 * @param  string $where
 * @return string       
 */
function wp_query_filter_max_one_year_old( $where = '' ) {
  $where .= " AND post_date > '" . date( 'Y-m-d', strtotime( '-356 days' ) ) . "'";
  return $where;
}

/**
 * Find similar posts based on tags.
 * 
 * @param  int  $post_id
 * @param  integer $limit          maximum number of posts
 * @param  integer $cache_duration time to cache in seconds. default: 1 week
 * @return array                   list of post_ids
 */
function get_similar_post_ids_by_tags( $post_id = NULL, $limit = 3, $cache_duration = 604800 ) {

	if ( ! $post_id )
		$post_id = get_the_ID();

	$cache_key = "inps_similar_" . $post_id;

	if ( false === ( $scores = get_transient( $cache_key ) ) ) {
	    
		$tags = wp_get_post_tags( $post_id );

		// fetch all last years posts with at least one matching tag
		$tag_ids = array_map( function( $t ) { return $t->term_id; }, $tags );
		add_filter( 'posts_where', 'wp_query_filter_max_one_year_old' );
		$args = array(
		  'tag__in'             => $tag_ids,
		  'post__not_in'        => array( $post_id ),
		  'posts_per_page'      => -1
		);
		$query_similar_posts = new WP_Query( $args );
		remove_filter( 'posts_where', 'wp_query_filter_max_one_year_old' );

		$scores = array();

		// count identical tags for each post
		while ( $query_similar_posts->have_posts() ) {
		  $p = $query_similar_posts->next_post();

		  $identical_tags = 0;
		  $this_tags = wp_get_post_tags( $p->ID );
		  foreach ( $this_tags as $t ) {
		    if ( in_array( $t->term_id, $tag_ids ) ) {
		      $identical_tags++;
		    }
		  }

		  if ( $identical_tags > 0 ) {
		    $scores[] = array( 'post_id' => $p->ID, 'score' => $identical_tags );
		  }
		}

		// sort by score
		usort( $scores, function ( $a, $b ) {
		  if ( $a[ 'score' ] == $b[ 'score' ] ) {
		    return 0;
		  }
		  return ( $a[ 'score' ] > $b[ 'score' ] ) ? -1 : 1;
		} );

		// take first $limit only
		$scores = array_slice( $scores, 0, $limit );

		// return without scores
		$scores = array_map( function( $p ){ return $p[ 'post_id' ]; }, $scores );

	    set_transient( $cache_key, $scores, $cache_duration );
	}

	return $scores;
}

?>
<!-- Example for Theme file -->
<?php $similar_posts = get_similar_post_ids_by_tags(); ?>
<strong><?php __( 'Related Posts' ) ?></strong>
<ul>
	<?php foreach ( $similar_posts as $similar_post_id ): ?>
		<li>
			<a href="<?php echo get_permalink( $similar_post_id ); ?>"><?php echo get_the_title( $similar_post_id ); ?></a>
		</li>
	<?php endforeach; ?>
</ul>