WordPressの検索結果にカスタムフィールドの値を追加する

function cf_search_join( $join ) {
	global $wpdb;
	if ( is_search() ) {
		$join .= ' LEFT JOIN ' . $wpdb->postmeta . ' ON ' . $wpdb->posts . '.ID = ' . $wpdb->postmeta . '.post_id ';
	}
	return $join;
}
add_filter( 'posts_join', 'cf_search_join' );

function cf_search_where( $where ) {
	global $wpdb;
	if ( is_search() ) {
		$where = preg_replace(
			"/\(\s*" . $wpdb->posts . ".post_title\s+LIKE\s*(\'[^\']+\')\s*\)/",
			"(" . $wpdb->posts . ".post_title LIKE $1) OR (" . $wpdb->postmeta . ".meta_value LIKE $1)", $where );

		// 特定のカスタムフィールドを検索対象から外す（※１）
//		$where .= " AND (" . $wpdb->postmeta . ".meta_key NOT LIKE 'number')";
//		$where .= " AND (" . $wpdb->postmeta . ".meta_key NOT LIKE 'zip')";
//		$where .= " AND (" . $wpdb->postmeta . ".meta_key NOT LIKE 'access')";
	}
	return $where;
}
add_filter( 'posts_where', 'cf_search_where' );

function cf_search_distinct( $where ) {
	global $wpdb;
	if ( is_search() ) {
		return "DISTINCT";
	}
	return $where;
}
add_filter( 'posts_distinct', 'cf_search_distinct' );

// 検索対象を『タイトルのみ』にする
function __search_by_title_only( $search, & $wp_query ) {
	global $wpdb;
	if ( empty( $search ) )
		return $search; // skip processing - no search term in query
	$q = $wp_query->query_vars;
	$n = !empty( $q[ 'exact' ] ) ? '' : '%';
	$search =
		$searchand = '';
	foreach ( ( array )$q[ 'search_terms' ] as $term ) {
		$term = esc_sql( like_escape( $term ) );
		$search .= "{$searchand}($wpdb->posts.post_title LIKE '{$n}{$term}{$n}')";
		$searchand = ' AND ';
	}
	if ( !empty( $search ) ) {
		$search = " AND ({$search}) ";
		if ( !is_user_logged_in() )
			$search .= " AND ($wpdb->posts.post_password = '') ";
	}
	return $search;
}
//add_filter( 'posts_search', '__search_by_title_only', 500, 2 );// （※２）

下記サイトを参考にしました。
WordPress内の検索対象にカスタムフィールドも適用する