$sessid = '';
while (strlen($sessid) < 32)
{
  $sessid .= mt_rand(0, mt_getrandmax());
}
// To make the session ID even more secure we'll combine it with the user's IP
$sessid .= $this->CI->input->ip_address();
$sessid = md5(uniqid($sessid, TRUE))

CREATE TABLE `ws_sessions` (
  `session_id` varchar(255) BINARY NOT NULL DEFAULT '',
  `session_expires` int(10) UNSIGNED NOT NULL DEFAULT '0',
  `session_data` text,
  PRIMARY KEY  (`session_id`)
) TYPE=InnoDB;

<?php
class session {
   // session-lifetime
   var $lifeTime;
   // mysql-handle
   var $dbHandle;
   function open($savePath, $sessName) {
       // get session-lifetime
       $this->lifeTime = get_cfg_var("session.gc_maxlifetime");
       // open database-connection
       $dbHandle = @mysql_connect("server","user","password");
       $dbSel = @mysql_select_db("database",$dbHandle);
       // return success
       if(!$dbHandle || !$dbSel)
           return false;
       $this->dbHandle = $dbHandle;
       return true;
   }
   function close() {
       $this->gc(ini_get('session.gc_maxlifetime'));
       // close database-connection
       return @mysql_close($this->dbHandle);
   }
   function read($sessID) {
       // fetch session-data
       $res = mysql_query("SELECT session_data AS d FROM ws_sessions
                           WHERE session_id = '$sessID'
                           AND session_expires > ".time(),$this->dbHandle);
       // return data or an empty string at failure
       if($row = mysql_fetch_assoc($res))
           return $row['d'];
       return "";
   }
   function write($sessID,$sessData) {
       // new session-expire-time
       $newExp = time() + $this->lifeTime;
       // is a session with this id in the database?
       $res = mysql_query("SELECT * FROM ws_sessions
                           WHERE session_id = '$sessID'",$this->dbHandle);
       // if yes,
       if(mysql_num_rows($res)) {
           // ...update session-data
           mysql_query("UPDATE ws_sessions
                         SET session_expires = '$newExp',
                         session_data = '$sessData'
                         WHERE session_id = '$sessID'",$this->dbHandle);
           // if something happened, return true
           if(mysql_affected_rows($this->dbHandle))
               return true;
       }
       // if no session-data was found,
       else {
           // create a new row
           mysql_query("INSERT INTO ws_sessions (
                         session_id,
                         session_expires,
                         session_data)
                         VALUES(
                         '$sessID',
                         '$newExp',
                         '$sessData')",$this->dbHandle);
           // if row was created, return true
           if(mysql_affected_rows($this->dbHandle))
               return true;
       }
       // an unknown error occured
       return false;
   }
   function destroy($sessID) {
       // delete session-data
       mysql_query("DELETE FROM ws_sessions WHERE session_id = '$sessID'",$this->dbHandle);
       // if session was deleted, return true,
       if(mysql_affected_rows($this->dbHandle))
           return true;
       // ...else return false
       return false;
   }
   function gc($sessMaxLifeTime) {
       // delete old sessions
       mysql_query("DELETE FROM ws_sessions WHERE session_expires < ".time(),$this->dbHandle);
       // return affected rows
       return mysql_affected_rows($this->dbHandle);
   }
}
$session = new session();
session_set_save_handler(array(&$session,"open"),
                         array(&$session,"close"),
                         array(&$session,"read"),
                         array(&$session,"write"),
                         array(&$session,"destroy"),
                         array(&$session,"gc"));
session_start();
// etc...
?>

// 如果客户端使用cookie,可直接传递session到page2.php
echo '<br /><a href="page2.php">page 2</a>';

// 如果客户端禁用cookie
echo '<br /><a href="page2.php?' . SID . '">page 2</a>';
/*
默认php5.2.1下,SID只有在cookie被写入的同时才会有值,如果该session
对应的cookie已经存在,那么SID将为(未定义)空
*/