From PHPBB用户手册

# Table: 'phpbb_privmsgs_to'
CREATE TABLE phpbb_privmsgs_to (
	msg_id mediumint(8) UNSIGNED DEFAULT '0' NOT NULL, 短信ID
	user_id mediumint(8) UNSIGNED DEFAULT '0' NOT NULL, 接收者用户ID ??
	author_id mediumint(8) UNSIGNED DEFAULT '0' NOT NULL, 作者用户ID
	pm_deleted tinyint(1) UNSIGNED DEFAULT '0' NOT NULL, 是否被删除
	pm_new tinyint(1) UNSIGNED DEFAULT '1' NOT NULL, 是否是新短信
	pm_unread tinyint(1) UNSIGNED DEFAULT '1' NOT NULL, 是否是未读短信
	pm_replied tinyint(1) UNSIGNED DEFAULT '0' NOT NULL, 是否已经回复
	pm_marked tinyint(1) UNSIGNED DEFAULT '0' NOT NULL, 是否被标记 ??
	pm_forwarded tinyint(1) UNSIGNED DEFAULT '0' NOT NULL, 是否被转发
	folder_id int(11) DEFAULT '0' NOT NULL, 短信隶属的邮件夹ID
	KEY msg_id (msg_id),
	KEY author_id (author_id),
	KEY usr_flder_id (user_id, folder_id)
) CHARACTER SET `utf8` COLLATE `utf8_bin`;