WordPressでプラグインを使用しないで文字列検索する

function custom_search( $search, $wp_query ) {
  global $wpdb;
  if ( ! isset( $wp_query->query_vars ) ) return $search;

  if ( isset( $wp_query->query_vars['s'] ) ) {
    // 検索ワードを文字列で取得する
    $search_words = esc_html(get_search_query());

    // 文字列による複数検索を可能するため「全角スペース」を「半角スペース」に変更する
    $search_words = str_replace('　', ' ', $search_words);

    // 文字列を配列にする
    $search_words = explode(' ', $search_words);
  
    // 検索SQL文実行
    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 );