session_set_save_handler的调用过程是如何样的

<?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 fine
if (isset($_SESSION['counter'])) {
    $_SESSION['counter']++;
} else {
    $_SESSION['counter'] = 1;
}
echo '<br/>SessionID: '. session_id() .'<br/>Counter: '. $_SESSION['counter'];