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    位置： php 中文手册 -> php 内置函数 -> php 会话函数session

    session_set_save_handler()

    (PHP 4, PHP 5, PHP 7)

    设置用户自定义会话存储函数

    说明

    session_set_save_handler(callable $open,callable $close,callable $read,callable $write,callable $destroy,callable $gc[,callable $create_sid[,callable $validate_sid[,callable $update_timestamp]]]): bool

    自 PHP 5.4 开始，可以使用下面的方式来注册自定义会话存储函数：

    session_set_save_handler(object $sessionhandler[,bool $register_shutdown=TRUE]): bool

    session_set_save_handler()设置用户自定义会话存储函数。如果想使用 PHP 内置的会话存储机制之外的方式，可以使用本函数。例如，可以自定义会话存储函数来将会话数据存储到数据库。

    参数

    本函数有 2 种原型：

    $sessionhandler

    实现了SessionHandlerInterface，SessionIdInterface和/或SessionUpdateTimestampHandlerInterface接口的对象，例如SessionHandler。自 PHP 5.4 之后可以使用。

    $register_shutdown

    将函数session_write_close()注册为register_shutdown_function()函数。

    或者
    open(string $savePath, string $sessionName)

    open 回调函数类似于类的构造函数，在会话打开的时候会被调用。这是自动开始会话或者通过调用session_start()手动开始会话之后第一个被调用的回调函数。此回调函数操作成功返回TRUE，反之返回FALSE。

    close()

    close 回调函数类似于类的析构函数。在 write 回调函数调用之后调用。当调用session_write_close()函数之后，也会调用 close 回调函数。此回调函数操作成功返回TRUE，反之返回FALSE。

    read(string $sessionId)

    如果会话中有数据，read 回调函数必须返回将会话数据编码（序列化）后的字符串。如果会话中没有数据，read 回调函数返回空字符串。

    在自动开始会话或者通过调用session_start()函数手动开始会话之后，PHP 内部调用 read 回调函数来获取会话数据。在调用 read 之前，PHP 会调用 open 回调函数。

    read 回调返回的序列化之后的字符串格式必须与$write回调函数保存数据时的格式完全一致。 PHP 会自动反序列化返回的字符串并填充$_SESSION超级全局变量。虽然数据看起来和serialize()函数很相似，但是需要提醒的是，它们是不同的。请参考：session.serialize_handler。

    write(string $sessionId, string $data)

    在会话保存数据时会调用$write回调函数。此回调函数接收当前会话 ID 以及$_SESSION中数据序列化之后的字符串作为参数。序列化会话数据的过程由 PHP 根据session.serialize_handler设定值来完成。

    序列化后的数据将和会话 ID 关联在一起进行保存。当调用$read回调函数获取数据时，所返回的数据必须要和传入$write回调函数的数据完全保持一致。

    PHP 会在脚本执行完毕或调用session_write_close()函数之后调用此回调函数。注意，在调用完此回调函数之后，PHP 内部会调用$close回调函数。

    Note:

    PHP 会在输出流写入完毕并且关闭之后才调用 write 回调函数，所以在 write 回调函数中的调试信息不会输出到浏览器中。如果需要在 write 回调函数中使用调试输出，建议将调试输出写入到文件。

    destroy($sessionId)

    当调用session_destroy()函数，或者调用session_regenerate_id()函数并且设置 destroy 参数为TRUE时，会调用此回调函数。此回调函数操作成功返回TRUE，反之返回FALSE。

    gc($lifetime)

    为了清理会话中的旧数据，PHP 会不时的调用垃圾收集回调函数。调用周期由session.gc_probability和session.gc_divisor参数控制。传入到此回调函数的 lifetime 参数由session.gc_maxlifetime设置。此回调函数操作成功返回TRUE，反之返回FALSE。

    create_sid()

    当需要新的会话 ID 时被调用的回调函数。回调函数被调用时无传入参数，其返回值应该是一个字符串格式的、有效的会话 ID。

    返回值

    成功时返回TRUE，或者在失败时返回FALSE。

    更新日志

    版本说明
    7.0.0新增可选参数$validate_sid和$update_timestamp。
    5.5.1新增可选参数$create_sid。
    5.4.0新增用来实现自定义会话处理器的接口SessionHandlerInterface；新增 PHP 内部会话处理器类：SessionHandler。

    范例

    自定义会话管理器：完整代码请参见SessionHandlerInterface。

    下列代码适用于 PHP 5.4.0 及以上版本。这里仅列出了调用方式，完整代码请参见SessionHandlerInterface。

    这里使用了session_set_save_handler()函数的 OOP 原型并且使用第二个参数来注册 shutdown 函数。当将对象注册为会话保存管理器时，建议使用这种方式。

    <?php
class MySessionHandler implements SessionHandlerInterface
{
    // 在这里实现接口
}
$handler = new MySessionHandler();
session_set_save_handler($handler, true);
session_start();
// 现在可以使用 $_SESSION 保存以及获取数据了

    使用对象自定义会话保存管理器

    下列代码适用于 PHP 5.4.0 之前的版本。

    下例演示了基于文件的会话数据存储，和 PHP 默认的$files存储器很相似。通过对此示例代码进行扩展，你可以很方便的实现使用数据库保存会话数据的功能。

    针对于 PHP 5.4.0 之前的版本，通过调用register_shutdown_function()函数来注册session_write_close()回调函数。这也是我们建议的方式。

    <?php
class FileSessionHandler
{
    private $savePath;
    function open($savePath, $sessionName)
    {
        $this->savePath = $savePath;
        if (!is_dir($this->savePath)) {
            mkdir($this->savePath, 0777);
        }
        return true;
    }
    function close()
    {
        return true;
    }
    function read($id)
    {
        return (string)@file_get_contents("$this->savePath/sess_$id");
    }
    function write($id, $data)
    {
        return file_put_contents("$this->savePath/sess_$id", $data) === false ? false : true;
    }
    function destroy($id)
    {
        $file = "$this->savePath/sess_$id";
        if (file_exists($file)) {
            unlink($file);
        }
        return true;
    }
    function gc($maxlifetime)
    {
        foreach (glob("$this->savePath/sess_*") as $file) {
            if (filemtime($file) + $maxlifetime < time() && file_exists($file)) {
                unlink($file);
            }
        }
        return true;
    }
}
$handler = new FileSessionHandler();
session_set_save_handler(
    array($handler, 'open'),
    array($handler, 'close'),
    array($handler, 'read'),
    array($handler, 'write'),
    array($handler, 'destroy'),
    array($handler, 'gc')
    );
// 下面这行代码可以防止使用对象作为会话保存管理器时可能引发的非预期行为
register_shutdown_function('session_write_close');
session_start();
// 现在可以使用 $_SESSION 保存以及获取数据了

    注释

    Warning

    在脚本执行完毕之后，PHP 内部会清除对象，所以有可能不调用$write和$close回调函数。这样可能会引发非预期的行为，所以当使用对象作为会话保存管理器时，需要通过注册 shutdown 回调函数来规避风险。通常，你可以通过调用register_shutdown_function()函数来注册'session_write_close'回调函数。

    在 PHP 5.4.0 中，可以调用session_register_shutdown()函数来注册 shutdown 回调函数。如果你使用session_set_save_handler()的 OOP 原型，那么仅需设置“register shutdown”为TRUE即可。

    Warning

    在 PHP 5.0.5 中，在对象销毁之后才会调用$write和$close回调函数，所以，在这两个回调函数中不可以使用对象，也不可以抛出异常。如果在函数中抛出异常，PHP 既不会捕获它，也不会跟踪它，这样会导致程序异常终止。但是对象析构函数可以使用会话。

    可以在析构函数中调用session_write_close()函数来解决这个问题。但是注册 shutdown 回调函数才是更加可靠的做法。

    Warning

    如果会话在脚本结束后关闭，对于某些 SAPI 而言，当前工作目录可能已经被改变。可以调用session_write_close()函数在脚本执行结束之前关闭会话。

    参见

    As of PHP 7.0, you can implement SessionUpdateTimestampHandlerInterface to 
define your own session id validating method like validate_sid and the timestamp updating method like update_timestamp in the non-OOP prototype of session_set_save_handler().
SessionUpdateTimestampHandlerInterface is a new interface introduced in PHP 7.0, which has not been documented yet. It has two abstract methods: SessionUpdateTimestampHandlerInterface :: validateId($sessionId) and SessionUpdateTimestampHandlerInterface :: updateTimestamp($sessionId, $sessionData).
<?php
  /*
    @author Wu Xiancheng
    Code structure for PHP 7.0+ only because SessionUpdateTimestampHandlerInterface is introduced in PHP 7.0
    With this class you can validate php session id and update the timestamp of php session data
    with the OOP prototype of session_set_save_handler() in PHP 7.0+
  */
  class PHPSessionXHandler implements SessionHandlerInterface, SessionUpdateTimestampHandlerInterface {
    public function close(){
      // return value should be true for success or false for failure
      // ...
    }
    public function destroy($sessionId){
      // return value should be true for success or false for failure
      // ... 
    }
    public function gc($maximumLifetime){
      // return value should be true for success or false for failure
      // ...
    }
    public function open($sessionSavePath, $sessionName){
      // return value should be true for success or false for failure
      // ...
    }
    public function read($sessionId){
      // return value should be the session data or an empty string
      // ...
    }
    public function write($sessionId, $sessionData){
      // return value should be true for success or false for failure
      // ...
    }
    public function create_sid(){
      // available since PHP 5.5.1
      // invoked internally when a new session id is needed
      // no parameter is needed and return value should be the new session id created
      // ...
    }
    public function validateId($sessionId){
      // implements SessionUpdateTimestampHandlerInterface::validateId()
      // available since PHP 7.0
      // return value should be true if the session id is valid otherwise false
      // if false is returned a new session id will be generated by php internally
      // ...
    }
    public function updateTimestamp($sessionId, $sessionData){
      // implements SessionUpdateTimestampHandlerInterface::validateId()
      // available since PHP 7.0
      // return value should be true for success or false for failure
      // ...
    }
  }
?>

    After spend so many time to understand how PHP session works with database and unsuccessful attempts to get it right, I decided to rewrite the version from our friend stalker.
//Database
CREATE TABLE `Session` (
 `Session_Id` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
 `Session_Expires` datetime NOT NULL,
 `Session_Data` text COLLATE utf8_unicode_ci,
 PRIMARY KEY (`Session_Id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8_unicode_ci;
SELECT * FROM mydatabase.Session;
<?php
//inc.session.php
class SysSession implements SessionHandlerInterface
{
  private $link;
  
  public function open($savePath, $sessionName)
  {
    $link = mysqli_connect("server","user","pwd","mydatabase");
    if($link){
      $this->link = $link;
      return true;
    }else{
      return false;
    }
  }
  public function close()
  {
    mysqli_close($this->link);
    return true;
  }
  public function read($id)
  {
    $result = mysqli_query($this->link,"SELECT Session_Data FROM Session WHERE Session_Id = '".$id."' AND Session_Expires > '".date('Y-m-d H:i:s')."'");
    if($row = mysqli_fetch_assoc($result)){
      return $row['Session_Data'];
    }else{
      return "";
    }
  }
  public function write($id, $data)
  {
    $DateTime = date('Y-m-d H:i:s');
    $NewDateTime = date('Y-m-d H:i:s',strtotime($DateTime.' + 1 hour'));
    $result = mysqli_query($this->link,"REPLACE INTO Session SET Session_Id = '".$id."', Session_Expires = '".$NewDateTime."', Session_Data = '".$data."'");
    if($result){
      return true;
    }else{
      return false;
    }
  }
  public function destroy($id)
  {
    $result = mysqli_query($this->link,"DELETE FROM Session WHERE Session_Id ='".$id."'");
    if($result){
      return true;
    }else{
      return false;
    }
  }
  public function gc($maxlifetime)
  {
    $result = mysqli_query($this->link,"DELETE FROM Session WHERE ((UNIX_TIMESTAMP(Session_Expires) + ".$maxlifetime.") < ".$maxlifetime.")");
    if($result){
      return true;
    }else{
      return false;
    }
  }
}
$handler = new SysSession();
session_set_save_handler($handler, true);
?>
<?php
//page 1
require_once('inc.session.php');
session_start();
$_SESSION['var1'] = "My Portuguese text: SOU Gaucho!";
?>
<?php
//page 2
require_once('inc.session.php');
session_start();
if(isset($_SESSION['var1']){
echo $_SESSION['var1']; 
}
//OUTPUT: My Portuguese text: SOU Gaucho!
?>

    If you have a custom handler registered ini_get('session.save_handler') will return 'user' instead of 'file'
    I had trouble with committing session data.
To "commit and continue" without closing your session, put this at the top of your "write" method:
<?php
$id = session_id();
session_write_close();
session_id($id);
session_start();
?>
Note that ANY time php generates a new session id, it is not automatically updated in a database. This can be helpful:
<?php
public function resetSessionId()
{
  $old = session_id();
  session_regenerate_id();
  $new = session_id();
  SessionHandler::regenerate_id($old,$new);
}
public function regenerate_id($old,$new)
{
  $db = mysqli->connect(...);
  $db->query('UPDATE sessions SET session_id = \''.$db->escape_string($new).'\' 
  WHERE session_id = \''.$db->escape_string($old).'\'');
}
?>

    Just a few words to explain some troubles while using session_set_save_handler(). It appears that internally PHP calls session management functions in this order: open(), read(), write(), close(). Close() function is called even if you does not make a call to sesison_start(), perhaps for some reasons like cleaning. 
  If you try to redefine these functions and call sessions_set_save_handler() but something doesn't work, (in my case the ssion data hasn't been written) it's a good idea to debug them in the order they are called. They doesn't produce error output to browser but you can use print or echo. 
  Shortly, if your write() function doesn't work as expected take a look for errors in previous functions - open() and read(). 
  I hope that this will help to save someone few hours debugging.
    Note that for security reasons the Debian and Ubuntu distributions of php do not call _gc to remove old sessions, but instead run /etc/cron.d/php*, which check the value of session.gc_maxlifetime in php.ini and delete the session files in /var/lib/php*. This is all fine, but it means if you write your own session handlers you'll need to explicitly call your _gc function yourself. A good place to do this is in your _close function, like this:
<?php
function _close() {
  _gc(get_cfg_var("session.gc_maxlifetime"));
  // rest of function goes here
}
?>

    The "binary" data that is in the session data appears to surround class/object names, and if you pass your session data through a function to sanitize it for SQL injection, you may indeed run in to problems.
For example, using the PDO::quote() function to prepare the data for injection (in my case for SQLite3), it was stopping short as soon as it encountered the first bit of binary data, causing my session information to be corrupted.
This change *must* have happened somewhere in the 5.2 series, because I just started encountering this problem recently on a code base that had been tested & working on earlier versions of PHP 5.2.
This may in fact be a bug - I have not yet checked... but beware, and perhaps using base64 to encode/decode your session data is a good thing to do just to be sure (though you are now left unable to visually inspect serialized session information at the storage level which is a rather big problem for on-the-fly debugging of sessions).
    For some people it might be important to know, that if the standard session handler has been overwritten with session_set_save_handler, no locking is working anymore (between session_read and session_write). The following might happen:
script "A" start     .
read session data    .
.            script "B" start
.            read session data
running (30secs)     add session data
.            write sesion data
.            script "B" stop
write session data    .
script "A" stop     .
If a script "A" runs for a long time (say 30secs) the same user might start another script "B", which also uses a session. Script "B" will start and read session data, even though script "A" is still running. Because script "B" is much faster it will finish its work and write back its session data before script "A" has ended. Now script "A" ends and overwrites all of script "B"'s session data. If you DON'T use session_set_save_handler, this cannot happend, because in this case, PHP will not start script "B" until script "A" ends.
    Note that if session.auto_start is set to On in the php.ini, your session_set_save_handler will return false as the session has already been initialized.
If you are finding that your code works OK on one machine but doesn't work on another, check to see if session.auto_start is set to On
    I write a class that unites whole handler functionality. It's not even needed to save instances of this class in variables. Just add a row:
<?php
new SessionSaveHandler();
?>
and the handler will rule the sessions ;-)
<?php
class SessionSaveHandler {
  protected $savePath;
  protected $sessionName;
  public function __construct() {
    session_set_save_handler(
      array($this, "open"),
      array($this, "close"),
      array($this, "read"),
      array($this, "write"),
      array($this, "destroy"),
      array($this, "gc")
    );
  }
  public function open($savePath, $sessionName) {
    $this->savePath = $savePath;
    $this->sessionName = $sessionName;
    return true;
  }
  public function close() {
    // your code if any
    return true;
  }
  public function read($id) {
    // your code
  }
  public function write($id, $data) {
    // your code
  }
  public function destroy($id) {
    // your code
  }
  public function gc($maxlifetime) {
    // your code
  }
}
new SessionSaveHandler();
?>

    session_set_save_handler is used before session_start.if your session is setted as auto start. it will return FALSE value.so you need add session_write_close() before session_set_save_handler to cancel the session's auto start.it likes this:
<?php
/*
qq:290359552
*/
session_write_close(); //cancel the session's auto start,important
function open()
{
  ...
}
....
session_set_save_handler( ... );
session_start();
?>

    I think it is very important here to stress that the WRITE method should use UPDATE+INSERT (or mysql specific REPLACE).
There is example code "out there" that uses just UPDATE for the write method, in which case, when session_regenerate_id is called, session data is lost (as an update would fail, as the key has changed).
I've just wasted a whole day due to this (I know I should have thought it through / RTFM, but it is an easy trap to fall into).
    It seems when you call 'session_name()', php loads the session id automatically from GET ( if the index exists ) and passes it to the 'read' callback method correctly, but the 'write' callback is invoked twice: first the auto-generated session id, then the custom session id
So be aware of what queries you execute inside the callback .. I got crazy because I used a MySQL 'REPLACE' statement to agilize, and I spent a lot of hours trying to understand why 2 rows instead of 1 were being affected ( the first id was inserting, the second updating )
I hope this helps!
    after spending 8 hours to find out whats going on..
just for the records, because php.net ignore the real world out there:
debian 5 installs by default the php-suhosin module, which changes the behavior of session_set_save_handler read/write function.
on calling the session write function the session data will be encrypted, and the returning string from the read function are decrypted and verified.
the encrypted data is no more compatible with session_encode/session_decode.
and breaks by default, subdomain handling and multiple host setups where different document roots are used.
for futher information look at:
http://www.hardened-php.net/suhosin/configuration.html
session sample data (debian 4):
test|s:3:"sdf";
session sample data (debian 5, with php-suhosin):
3GdlPEGr2kYgRFDs-pUSoKomZ4fN7r5BM5oKOCMsWNc...
i thing the suhosin patch should report a warning in case of invalid session data, to get a clue whats going wrong.
    object- and mysql-based session-handler, requires the following table:
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...
?>

    Note that as well as destructing objects before calling write() and close(), it seems PHP also destroys classes. That is, you can't even call a static method of an external class in the write() and close() handlers - PHP will issue a Fatal error stating "Class xxxx not found"
    When storing sessions in a DB, it's usually beneficial to use an existing custom DB object, but this creates problems with the latest version of PHP 5.3.1. This used to work fine on PHP 5.2.x (Linux and Windows).
The problem now is that session_write_close() is not automatically called when execution ends, but rather after all the objects have been destructed, including the DB object!
There are two ways around this, either manually calling session_write_close() at the end of your script(s), or not using the DB object.
I'm sure this is the intended behavior from the beginning.
    When writing your own session handler, particularly database session handlers, play close attention to garbage cleanup and how it could affect server load.
To pick a round number example:
If you have 1000 requests per minute on session enabled pages, everyone needs a session started but the session garbage cleanup does not need to run every request. Doing so would cause unrequired queries on the database server.
In this example, setting your probability/divisor to 1/1000 would be sufficient to clean up old sessions at a minimum once a minute. If you don't need that kind of granularity, increase the gc divisor.
Finding the tradeoff between clearing up old sessions and server load is the important aspect here.
    It is important to understand that PHP's default file-based session handling LOCKS the session file, inherently allowing ONLY ONE thread handling any given session at a time.
When you implement a DB-backed session storage and you do not do any locking, you may run into situations where more than one thread is serving the same session, and you may LOSE DATA because the second thread will overwrite any session changes done by the first thread.
You should therefore think about locking the session somehow if you want to have the exact same behavior as with the default file-based implementation. For example, with InnoDB you could do a SELECT ... FOR UPDATE, or you can use the GET_LOCK() function.
    If saving to a database, as in the examples on this page, for performance, consider the following.
Build the Sessions table with an index on the SessionExpires column to quickly identify rows to be deleted in the garbage collection phase.
Rather do a garbage collection "delete from sessions where expiresOn < $now" on every session start/open. If you have an index on expiry time, this will not be a big hit, and evens out the load across all users. If it is possible that a large number of sessions will expire at the same time, include a "limit 100" clause, set for whatever number is reasonable, so that each user shares the load.
Use a varchar rather than Text to store the data, as Text will store the column off-page and is retrieved slightly slower. Use Text only if your application really does store large amounts of text in the session.
    Adding to the very useful class from: andreipa at gmail dot com
1. You should handle session expiration & data I/O from the SessionHandlerInterface methods,
2. You should NOT handle session regeneration and data modification from these methods but from a static method, e.g. sth like Session::start().
3. PHP gives a lot of examples but does NOT say what's the perspective under which one should work.
A skeleton of such a class:
namespace xyz;
class Session implements \SessionHandlerInterface, Singleton {
  /** @var SessionToken $token The SessionToken of this command; 
     this is part of my programming approach */
  protected $token;
  /** @var PDO $dbh The PDO handler to the database */
  protected $dbh;
  /** @var $savePath Where sessions are stored */
  protected $savePath;
  /** @var $type Type of sessions (['files'|'sqlite']) */
  protected $type;
  /** @var self $instance An instance of this class */
  static private $instance = null;
  private function __construct() { ... }
  static public function getInstance() {
   if (self::$instance === null) {
     self::$instance = new self();
   }
   return self::$instance;
  }
  public function open($savePath, $sessionName) { ... }
  public function close() {
   if ($this->type == static::FILES) {
     return true;
   } elseif ($this->type == static::SQLITE) {
     return true;
   }
  }
  public function read($id) { ... }
  public function write($id, $data) { ... }
  public function destroy($id) { ... }
  public function gc($maxlifetime) { ... }
  static public function get($key) {
   return (isset($_SESSION[$key]))? $_SESSION[$key] : null;
  }
  static public function set($key, $value) {
   return $_SESSION[$key] = $value;
  }
  static public function newId() {...}
  static public function start($call = null, $log = false) {
   //1. start session (send 1st header)
   if (session_status() != PHP_SESSION_ACTIVE) {
     session_start();  //calls: open() >read()
   }
   //2. $_SESSION['session']: array of session control data
   // existed session
   if (is_array(static::get('session'))) {
     $session = static::get('session');
   // new session
   } else {
     $session = array();
   }
   $tmp = $_SESSION;
   //do sth with $session array...
   static::set('session', $session);
   session_write_close();  //calls: write() >read() >close()
   //create a new session inside if...else...
   session_id(static::newId());
   session_start();  //calls: open() >read()
   //if you want previous session data to be copied:
   //$_SESSION = $tmp;
   //do sth else with $session array and save it to new session...
   static::set('session', $session);
   //6. call callback function (only on valid/new sessions)
   if ($call)
     $call();
   session_write_close();  //calls: write() >read() >close()
  }
  /**
  * Defines custom session handler.
  */
  static public function setHandler() {
   // commit automatic session
   if (ini_get('session.auto_start') == 1) {
     session_write_close();
   }
   $handler = static::getInstance();
   session_set_save_handler($handler, true);
  }
}
Let's start a session:
Session::setHandler();
Session::start();
Trying for hours to trace my error where the 3rd Session::read() ended to use a null Session::dbh until I realized that Session::close() should NOT destroy properties of this class!
Also I avoid the use of session_create_id() as it's only for PHP 7 >= 7.1.0 and I use in place a static Session::newId().
    Your custom session handler should not contain calls to any of the session functions, such as session_name() or session_id(), as the relevant values are passed as arguments on various handler methods. Attempting to obtain values from alternative sources may not work as expected.
    What is not documented is that callables $validate_sid and $update_timestamp are supported since PHP 7.0. for the 
 prototype of "bool session_set_save_handler ( callable $open , callable $close , callable $read , callable $write , callable $destroy , callable $gc [, callable $create_sid [, callable $validate_sid [, callable $update_timestamp ]]] )".
validate_sid($sessionId)
 This callback is to validate $sessionId. Its return value should be true for valid session id $sessionId or false for invalid session id $sessionId. If false is returned, a new session id is generated to replace the invalid session id $sessionId.
update_timestamp($sessionId)
 This call back is to update timestamp, and its return value should be true for success or false for failure.
If you use this prototype, if you provide less than 6 parameters or if you provide more parameters than session_set_save_handler() accepts, you will get a "Wrong parameter count for session_set_save_handler()" warning.
If you use the OOP prototype of session_set_save_handler(SessionHandlerInterface $sessionhandler [, bool $register_shutdown = true ] ), a member method named neither validate_sid nor update_timestamp of the class of $sessionhandler are not invoked even in PHP 7.2, but a member method named create_sid is supported as of PHP 5.5.1.
It's 16th December, 2017 today, the documetation even PHP may get updated sometime afterwards.
    Here is a class to handle session using an Oracle table.
https://github.com/shanikawm/PHP_Oracle_Based_Session_Handler_Class
<?php
/**
 * By Shanika Amarasoma
 * Date: 6/24/2016
 * PHP session handler using Oracle database
 * Oracle Create table statement
    CREATE TABLE PHP_SESSIONS
    (
      SESSION_ID VARCHAR2(256 BYTE) UNIQUE,
      DATA    CLOB,
      TOUCHED   NUMBER(38)
    );
 */
class session_handler implements SessionHandlerInterface
{
  private $con;
  public function __construct() {
    if(!$this->con=oci_pconnect(DBUSER,DBPASS,CONNECTION_STR)){
      die('Database connection failed !');
    }
  }
  public function open($save_path ,$name){
    return true;
  }
  public function close(){
    return true;
  }
  public function read($session_id){
    $query = "SELECT \"DATA\" FROM PHP_SESSIONS WHERE SESSION_ID=Q'{" . $session_id . "}'";
    $stid = oci_parse($this->con, $query);
    oci_execute($stid, OCI_DEFAULT);
    $row = oci_fetch_array($stid, OCI_ASSOC + OCI_RETURN_LOBS);
    oci_free_statement($stid);
    return $row['DATA'];
  }
  public function write($session_id,$session_data){
    $dquery="DELETE FROM PHP_SESSIONS WHERE SESSION_ID=Q'{".$session_id."}'";
    $dstid = oci_parse($this->con,$dquery);
    oci_execute($dstid, OCI_NO_AUTO_COMMIT);
    oci_free_statement($dstid);
    $query="INSERT INTO PHP_SESSIONS(SESSION_ID,TOUCHED,\"DATA\") VALUES(Q'{".$session_id."}',".time().",EMPTY_CLOB()) RETURNING \"DATA\" INTO :clob";
    $stid = oci_parse($this->con,$query);
    $clob=oci_new_descriptor($this->con,OCI_D_LOB);
    oci_bind_by_name($stid, ':clob', $clob, -1, OCI_B_CLOB);
    if(!oci_execute($stid, OCI_NO_AUTO_COMMIT)){
      @oci_free_statement($stid);
      return false;
    }
    if($clob->save($session_data)){
      oci_commit($this->con);
      $return=true;
    } else {
      oci_rollback($this->con);
      $return=false;
    }
    $clob->free();
    oci_free_statement($stid);
    return $return;
  }
  public function destroy($session_id){
    $query="DELETE FROM PHP_SESSIONS WHERE SESSION_ID=Q'{".$session_id."}'";
    $stid = oci_parse($this->con,$query);
    oci_execute($stid, OCI_DEFAULT);
    $rows=oci_num_rows($stid);
    oci_commit($this->con);
    oci_free_statement($stid);
    if($rows>0){
      return true;
    } else {
      return false;
    }
  }
  public function gc($maxlifetime){
    $query="DELETE FROM PHP_SESSIONS WHERE TOUCHED<".(time() $maxlifetime);
    $stid = oci_parse($this->con,$query);
    oci_execute($stid, OCI_DEFAULT);
    $rows=oci_num_rows($stid);
    oci_commit($this->con);
    oci_free_statement($stid);
    if($rows>0){
      return true;
    } else {
      return false;
    }
  }
}
session_set_save_handler(new session_handler(), true);
session_start();
    Persisting PHP sessions into mongodb (allows NLB without affinity)
https://gist.github.com/dimzon/62eeb9b8561bcb9f0c6d
    i dont see any mention of what happens when eg "open" calls "die", like mentioned in docs for "register_shutdown_function":
"If you call exit() within one registered shutdown function, processing will stop completely and no other registered shutdown functions will be called."
http://php.net/manual/en/function.register-shutdown-function.php
my result: same behavior--"read" will not get called if "open" calls "die"/"exit".
    I'm not using objects for the save handlers I'm using functions but still get weird behaviour with session writing not being called.
This fixes the problem though:
register_shutdown_function('session_write_close');
    I was getting Fatal error: Exception thrown without a stack frame and it took days to figure out the reason. I am using memcache to store sessions and in my custom class I use Memcache class in write method.
I put the code in the write method inside try-catch block and it solved my problem.
    You can call the session_write in db object destructor to be shore that you still have a connection to mysql and the session is write.
    I pulled a really stupid move. If you are trying to debug your garbage collector, make sure you call the following >>> BEFORE <<< "session_start":
<?php
ini_set('session.gc_probability', 100);
ini_set('session.gc_divisor', 100);
?>
I was sure it was a bug in PHP, but turned out (like 99% of the time) to be me own fault.
    if you simply append the information from session variables every time you'll have many multiples for variables each time they are changed. a simple way to do this is explode the data twice to seperate the variable name from the other relevant information and foreach() check against the stored set. here is a little bit of a mess i wrote to do it.
assuming stored session variables in both database and passed through function:
<?php
$buffer = array();
$buffer = explode('|',$sessiondata);
$buf1 = array();
$buf2 = array();
$finalbuff = '';
foreach($buffer as $i){ 
  $i = explode(';',$i); 
  foreach($i as $b){ 
    array_push($buf1,$b);
  }
}
$buffer = explode('|',$result['data']);
foreach($buffer as $i){ $i = explode(';',$i); foreach($i as $b){ array_push($buf2,$b);}}
$z = 0;
$x = 0;
while($buf2[$z]){
  while($buf1[$x]){
    if($buf2[$z] == $buf1[$x]){
      $buf2[($z+1)] = $buf1[($x+1)];
    }
    $x+=2;
  }
  $z+=2;
}
foreach($buf2 as $i){ $finalbuff .= $i; }
?>
$sessiondata is the variable passed through the function and $result['data'] is the data stored in an sql database.
    Regarding the SAPIs: The warning mentioned in function's description (that the Current working directory is changed with some SAPIs) is very important.
It means that if your callback 'write' function needs to write to a file in current directory, it will not find it. You have to use absolute path and not rely upon the current working directory.
I thought this warning applies only to some strange environments like Windows, but it happens exactly on Linux + Apache 2.2 + PHP 5.
    It is useful to use MEMORY storage engine in MySQL while handling sessions.
http://dev.mysql.com/doc/refman/5.0/en/memory-storage-engine.html
    When using a custom session handler, if the first callback function (sessOpen in my case) finds no session id, one is set by the time the second argument (sessRead in my case) is called.
    the behavior, return values, and exact time of calling for these functions is pretty poorly documented here. i thought folks might like to know that:
1) calling session_start() triggers PHP to first call your open function and then call your read function before resuming with the code immediately following your session_start() call.
2) calling session_id('some_value') within your open function WILL NOT SET THE SESSION COOKIE (at least not on my setup - PHP 4.4.1). Assuming you defined some function to validate a session id called my_func(), you might want to do something like this in your open function:
<?php
 function _open($save_path, $session_name) {
  // check for session id
  $sess_id = session_id();
  if (empty($sess_id) || !myfunc($sess_id)) {
   //session_id is INVALID - generating new
   $new_id = md5(uniqid("some random seed here"));
   session_id($new_id);
   setcookie(session_name(),
          $new_id,
          0,
          "/",
          ".mydomain.com");
   }
  return true;
 } // _open()
?>

    If a session is closed the save-handlers seem to be resetted (in PHP 4.1.2 they are), so you need to run session_set_save_handler() again after e.g. running session_write_close() and restarting the session with session_start();
    You can't use the session autostart feature with 
session.save_handler = user
set in your php.ini. Use instead the auto_prepend_file directive in the php.ini and point it to your save_handler with an session_start() at the end.
    With regards to the read handler, the docs say:
 "Read function must return string value always to make save 
 handler work as expected. Return empty string if there is no 
 data to read."
I can't emphasize this enough. I just spent half a day trying to figure out why my sessions weren't storing any information. I was blithely returning the results of a query on the database from the read handler. Since there was no match for the new ID, the result was NULL. Since it wasn't a string, sessions were essentially disabled. So, the safe thing might be something like this:
<?php
function sessRead($id)
{
  // Look up data
  $results = getStuff($id);
  
  // Make sure it's a string
  settype($results, 'string');
  return $results;
}
?>
Of course, you can do whatever you want with it. But, no matter what, make sure you return a string.
HTH,
Michael
    When using mySQL for your session handling functions, don't forget to call mysql_select_db() to change the database if you are using a separate database for your session data. Call mysql_select_db() INSIDE every handler function that accesses the database, since if you write session data after accessing another database, it will not change the database to your session database, and therefore, not write the session data.
    Below is a session id value validator I just wrote. It is especially important to validate session id cookie values when using a custom file based validator, otherwise hackers could potentially trick it into overwriting non-session files.
/**
* Validates the value (the session id) of a session cookie.
* Useful for detecting potential hack attempts.
* It is up to the caller to delete the cookie if necessary.
* See also: http://lxr.php.net/xref/PHP_TRUNK/ext/session/session.c#php_session_valid_key
*
* @param string $value
* @param boolean $debug
* @return boolean
*/
function session_validate($cookie_value, $debug = false) {
  // session.hash_function allows you to specify the hash algorithm used to generate the session IDs. '0' means MD5 (128 bits) and '1' means SHA-1 (160 bits). Since PHP 5.3.0 it is also possible to specify any of the algorithms provided by the hash extension (if it is available), like sha512 or whirlpool. A complete list of supported algorithms can be obtained with the hash_algos() function.
  // session.hash_bits_per_character allows you to define how many bits are stored in each character when converting the binary hash data to something readable. The possible values are '4' (0-9, a-f), '5' (0-9, a-v), and '6' (0-9, a-z, A-Z, "-", ",").
  if (!(isset($cookie_value) && is_string($cookie_value) && strlen($cookie_value))) {
    return false;
  }
  $bits = null;
  if (1) {
    $hash_function = ini_get('session.hash_function');
    $hash_function_to_bits = array(
      0  => 128,
      1  => 160,
    );
    $bits = @$hash_function_to_bits[$hash_function];
  }
  $bits_per_char = ini_get('session.hash_bits_per_character');
  $bits_per_char_to_charclass = array(
    4  => '0-9a-f',
    5  => '0-9a-v',
    6  => '0-9a-zA-Z\-,',  // this is also the default
  );
  $charclass = array_key_exists($bits_per_char, $bits_per_char_to_charclass) ? $bits_per_char_to_charclass[$bits_per_char] : $bits_per_char_to_charclass[6];
  $charlength = $bits ? (integer)ceil($bits / $bits_per_char) : '1,128'; // the last value is a somewhat arbitrary default
  $re = '/^[' . $charclass . ']{' . $charlength . '}$/';
  $result = preg_match($re, $cookie_value);
  $debug && error_log(__FUNCTION__ . ' regexp: ' . $re . "\tresult: " .intval($result));
  return $result;
}

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