WordPress - get related posts by post tags

single.php

// Add this to your single.php or somewhere else where you see fit. :)
//
$post_id = get_the_ID();

// Extract post tags
$tags = get_the_terms( $post_id, 'post_tag' );
$tag_list = wp_list_pluck( $tags, 'term_id' );
$tag_list = implode( ',', $tag_list );

// Select post ID's from term_relationships table and order by those that match the most number of tags
$query = "SELECT DISTINCT object_id, COUNT(object_id) AS hit_count
         FROM {$wpdb->prefix}term_relationships
         WHERE object_id <> {$post_id} AND term_taxonomy_id IN ({$tag_list}) GROUP BY object_id ORDER BY hit_count DESC";

$posts = $wpdb->get_results( $query, ARRAY_A );
$post_list = wp_list_pluck( $posts, 'object_id' );

// Make a standard WP_Query and get relevant posts
$args = array(
    'post_type' => 'post',
    'post_status' => 'publish',
    'post__not_in' => array( $post_id ), // Redundant, but you can never be too sure, right? ;)
    'post__in' => $post_list,
    'orderby' => 'rand',
    'date_query' => array(
        'after' => date('Y-m-d', strtotime('-1 year'))
    )
);

$related = new WP_Query( $args );
// Perform the loop magic