/**
 * 検索にタイトル・本文以外にターム名・カスタムフィールド・ユーザー名をor検索にする
 *
 * @param Search   $search search.
 * @param Wp_query $wp_query search.
 * @return Search
 * */
function custom_search( $search, $wp_query ) {
    global $wpdb;
    // 検索画面でなければ戻る.
    if ( ! isset( $wp_query->query_vars ) ) {
        return $search;
    }

    if ( isset( $wp_query->query_vars['s'] ) ) {
        // カスタムフィールド・ターム名・ユーザー名を検索対象にする.
        $search_words = explode( ' ', isset( $wp_query->query_vars['s'] ) ? $wp_query->query_vars['s'] : '' );

        if ( count( $search_words ) > 0 ) {
            $search = '';
            foreach ( $search_words as $word ) {
                if ( ! empty( $word ) ) {
                    $search_word        = '%' . esc_sql( $word ) . '%';
                    $posts              = esc_sql( $wpdb->posts );
                    $users              = esc_sql( $wpdb->users );
                    $term_relationships = esc_sql( $wpdb->term_relationships );
                    $term_taxonomy      = esc_sql( $wpdb->term_taxonomy );
                    $terms              = esc_sql( $wpdb->terms );
                    $postmeta           = esc_sql( $wpdb->postmeta );
                    $search     .= " AND (
                        {$posts}.post_title LIKE '{$search_word}'
                        OR {$posts}.post_content LIKE '{$search_word}'
                        OR {$posts}.post_author IN (
                            SELECT distinct ID
                            FROM {$users}
                            WHERE display_name LIKE '{$search_word}'
                        )
                        OR {$posts}.ID IN (
                            SELECT distinct r.object_id
                            FROM {$term_relationships} AS r
                            INNER JOIN {$term_taxonomy} AS tt ON r.term_taxonomy_id = tt.term_taxonomy_id
                            INNER JOIN {$terms} AS t ON tt.term_id = t.term_id
                            WHERE t.name LIKE '{$search_word}'
                            OR t.slug LIKE '{$search_word}'
                            OR tt.description LIKE '{$search_word}'
                        )
                        OR {$posts}.ID IN (
                            SELECT distinct p.post_id
                            FROM {$postmeta} AS p
                            WHERE p.meta_value LIKE '{$search_word}'
                        )
                    ) ";
                }
            }
        }
        return $search;
    }
}
add_filter( 'posts_search', 'custom_search', 10, 2 );