一个开发环境有多个网站，需要使用不同的session，解决方案很多。不过这次也高大上一把，用数据库存，方便以后扩展。

PostgreSQL版
首先是数据库的部分

--drop table php_session
create unlogged table php_session
(
  sess_id varchar(32) primary key,
  modify_time timestamp with time zone not null,
  sess_data varchar(3000) default ''
);

create index concurrently idx_php_session_modify_time on php_session(modify_time);

--set_session(id, data)
create or replace function set_session(varchar, varchar) returns void as $set_session$
  with upsert as (
    update php_session
    set modify_time = current_timestamp, sess_data = $2
    where sess_id = $1
    returning 1
  )
  insert into php_session (sess_id, modify_time, sess_data)
  select $1, current_timestamp, $2
  where not exists (
    select 1 from upsert
  );
$set_session$ language sql;

--get_session(id)
create or replace function get_session(varchar) returns varchar as $get_session$
  select sess_data from php_session where sess_id = $1
$get_session$ language sql;

--del_session
create or replace function del_session(varchar) returns void as $del_session$
  delete from php_session where sess_id = $1
$del_session$ language sql;

--gc_session
create or replace function gc_session() returns void as $del_session$
  delete from php_session where modify_time < current_timestamp - interval '30 days'
$del_session$ language sql;

然后是PHP的部分

<?php

session_set_save_handler(
  function ($savePath, $sessionName) {//open
    return true;
  },
  function () {//close
    return true;
  },
  function ($id) {//read
    $sql = "select get_session($1)";
    $stmt = pg_query_params(SESSION_CONN, $sql, array($id));
    $result = pg_fetch_row($stmt);
    
    return $result[0];
  },
  function ($id, $data) {//write
    $sql = "select set_session($1, $2)";
    pg_query_params(SESSION_CONN, $sql, array($id, $data));
    return true;
  },
  function ($id) {//destroy
    $sql = "select del_session($1)";
    pg_query_params(SESSION_CONN, $sql, array($id, $data));
    return true;
  },
  function ($maxlifetime) {//gc
    //php needn't control the global session gc 
    return true;
  }
);

register_shutdown_function('session_write_close');

?>

然后只要在session_start之前调用这个就可以了

至于SESSION_CONN，那是我定义的一个常量，表示一个指向session数据库的链接而已。

MySQL版
再总结一个针对MySQL的集成更多基础功能的例子：
表结构：

 CREATE TABLE IF NOT EXISTS `sessioninfo` (
 `sid` varchar(255) NOT NULL,
 `value` text NOT NULL,
 `expiration` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
 PRIMARY KEY (`sid`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

session信息存储到数据库的类：

class MySessionHandler implements SessionHandlerInterface {
 
  /**
  * @access private
  * @var object 数据库连接
  */
  private $_dbLink;
  /**
  * @access private
  * @var string 保存session的表名
  */
  Private $_sessionTable;
  /**
  * @access private
  * @var string session名
  */
  private $_sessionName;
  /**
  * @const 过期时间
  */
  const SESSION_EXPIRE = 10;
 
  public function __construct($dbLink, $sessionTable) {
    if(!is_object($dbLink)) {
      return false;
    }
    $this->_dbLink = $dbLink;
    $this->_sessionTable = $sessionTable;
  }
 
  /**
  * 打开
  * @access public
  * @param string $session_save_path 保存session的路径
  * @param string $session_name session名
  * @return integer
  */
  public function open($session_save_path, $session_name) {
    $this->_sessionName = $session_name;
    return 0;
  }
 
  /**
  * 关闭
  * @access public
  * @return integer
  */
  public function close() {
    return 0;
  }
 
  /**
  * 关闭session
  * @access public
  * @param string $session_id session ID
  * @return string
  */
  public function read($session_id) {
    $query = "SELECT value FROM {$this->_sessionTable} WHERE sid = {$session_id} AND UNIX_TIMESTAMP(expiration) + " . self::SESSION_EXPIRE . " > UNIX_TIMESTAMP(NOW())";
    $result = $this->_dbLink->query($query);
    if(!isset($value) || empty($value)) {
      $value = "";
      return $value;
    }
    $this->_dbLink->query("UPDATE {$this->_sessionTable} SET expiration = CURRENT_TIMESTAMP() WHERE sid = {$session_id}");
    $value = $result->fetch_array();
    $result->free();
    return $value['value'];
  }
 
  /**
  * 写入session
  * @access public
  * @param string $session_id session ID
  * @param string $session_data session data
  * @return integer
  */
  public function write($session_id, $session_data) {
    $query = "SELECT value FROM {$this->_sessionTable} WHERE sid = '{$session_id}' AND UNIX_TIMESTAMP(expiration) + " . self::SESSION_EXPIRE . " > UNIX_TIMESTAMP(NOW())";
    $result = $this->_dbLink->query($query);
    $result = $result->fetch_array();
    if(!empty($result)) {
      $result = $this->_dbLink->query("UPDATE {$this->_sessionTable} SET value = {$session_data} WHERE sid = {$session_id}");
    }
    else{
      $result = $this->_dbLink->query("INSERT INTO {$this->_sessionTable} (sid, value) VALUES ('{$session_id}', '{$session_data}')");
    }
    if($result){
      return 0;
    }
    else{
      return 1;
    }    
  }
 
  /**
  * 销魂session
  * @access public
  * @param string $session_id session ID
  * @return integer
  */
  public function destroy($session_id) {
    $result = $this->_dbLink->query("DELETE FROM {$this->_sessionTable} WHERE sid = '{$session_id}'");
    if($result){
      return 0;
    }
    else{
      return 1;
    }
  }
 
  /**
  * 垃圾回收
  * @access public
  * @param string $maxlifetime session 最长生存时间
  * @return integer
  */
  public function gc($maxlifetime) {
    $result = $this->_dbLink->query("DELETE FROM {$this->_sessionTable} WHERE UNIX_TIMESTAMP(expiration) < UNIX_TIMESTAMP(NOW()) - " . self::SESSION_EXPIRE);
    if($result){
      return 0;
    }
    else{
      return 1;
    }
  }
 
}

$dbLink = new mysqli("localhost", "root", "root", "test");
$sessionTable = "sessioninfo";
 
$handler = new MySessionHandler($dbLink, $sessionTable);
session_set_save_handler($handler);
session_start();
$_SESSION['name'] = "test";
echo $_SESSION["name"];
//session_destroy();