session_set_save_handler的调用过程是如何样的

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/--><?php/**CREATE TABLE IF NOT EXISTS `sessions` (  `session` varchar(255) character set utf8 collate utf8_bin NOT NULL,  `session_expires` int(10) unsigned NOT NULL default '0',  `session_data` text collate utf8_unicode_ci,  PRIMARY KEY  (`session`)) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci; */class Session {    /**     * a database connection resource     * @var resource     */    private static $_sess_db;    /**     * Open the session     * @return bool     */    public static function open() {                if (self::$_sess_db = mysql_connect('localhost', 'root', '')) {            return mysql_select_db('my_application', self::$_sess_db);        }        return false;    }    /**     * Close the session     * @return bool     */    public static function close() {        return mysql_close(self::$_sess_db);    }    /**     * Read the session     * @param int session id     * @return string string of the sessoin     */    public static function read($id) {        $id = mysql_real_escape_string($id);        $sql = sprintf("SELECT `session_data` FROM `sessions` WHERE `session` = '%s'", $id);        if ($result = mysql_query($sql, self::$_sess_db)) {            if (mysql_num_rows($result)) {                $record = mysql_fetch_assoc($result);                return $record['session_data'];            }        }        return '';    }    /**     * Write the session     * @param int session id     * @param string data of the session     */    public static function write($id, $data) {        $sql = sprintf("REPLACE INTO `sessions` VALUES('%s', '%s', '%s')",                       mysql_real_escape_string($id),                       mysql_real_escape_string(time()),                       mysql_real_escape_string($data)                       );        return mysql_query($sql, self::$_sess_db);    }    /**     * Destoroy the session     * @param int session id     * @return bool     */    public static function destroy($id) {        $sql = sprintf("DELETE FROM `sessions` WHERE `session` = '%s'", $id);        return mysql_query($sql, self::$_sess_db);    }    /**     * Garbage Collector     * @param int life time (sec.)     * @return bool     * @see session.gc_divisor      100     * @see session.gc_maxlifetime 1440     * @see session.gc_probability    1     * @usage execution rate 1/100     *        (session.gc_probability/session.gc_divisor)     */    public static function gc($max) {        $sql = sprintf("DELETE FROM `sessions` WHERE `session_expires` < '%s'",                       mysql_real_escape_string(time() - $max));        return mysql_query($sql, self::$_sess_db);    }}//ini_set('session.gc_probability', 50);ini_set('session.save_handler', 'user');session_set_save_handler(array('Session', 'open'),                         array('Session', 'close'),                         array('Session', 'read'),                         array('Session', 'write'),                         array('Session', 'destroy'),                         array('Session', 'gc')                         );if (session_id() == "") session_start();//session_regenerate_id(false); //also works fineif (isset($_SESSION['counter'])) {    $_SESSION['counter']++;} else {    $_SESSION['counter'] = 1;}echo '<br/>SessionID: '. session_id() .'<br/>Counter: '. $_SESSION['counter'];