From PHPBB用户手册

# Table: 'phpbb_privmsgs_folder'
CREATE TABLE phpbb_privmsgs_folder (
	folder_id mediumint(8) UNSIGNED NOT NULL auto_increment, 邮件夹ID
	user_id mediumint(8) UNSIGNED DEFAULT '0' NOT NULL, 邮件夹隶属的用户ID
	folder_name varchar(255) DEFAULT '' NOT NULL, 邮件夹名称
	pm_count mediumint(8) UNSIGNED DEFAULT '0' NOT NULL, 内含邮件数目
	PRIMARY KEY (folder_id),
	KEY user_id (user_id)
) CHARACTER SET `utf8` COLLATE `utf8_bin`;