Understand PHP's SESSION mechanism

The main content of this article is about understanding the SESSION mechanism of PHP. It has a certain reference value. Now I share it with you. Friends in need can refer to it.

Completely understand the SESSION mechanism of PHP

1. The default mechanism uses disk files to implement PHP sessions. php.ini configuration: session.save_handler = files
　1 , session_start()
A. session_start() is the beginning of the session mechanism, it has a certain probability of starting garbage collection, Because the session is stored in the file,
PHP's own garbage collection is invalid. SESSION recycling requires deleting the file. This probability is based on php. It is determined by the configuration of ini,
But some systems are session.gc_probability =0, which means that the probability is 0, but garbage collection is implemented through cron scripts.
## stession
. GC_PROBABILITY ##= SESSION #.
gc_pisor =1000 session.
gc_maxlifetime =1440//The expiration time defaults to 24 minutes                            //The probability is session.gc_probability/session.gc_pisor result 1/1000,                 // It is not recommended to set it too small, because the garbage collection of the session needs to check whether each file has expired.

## session
.save_path =//It seems that different systems have different defaults. One setting is "N;/path"​​​​ //This is random hierarchical storage. In this case, garbage collection will not work and you need to write your own script

B. The session will determine whether there is currently
$_COOKIE[session_name()];session_name() returns the COOKIE key value that saves the session_id, This value can be found from php.ini
# PHPSESSID
//Default valuePHPSESSID##                    , If it does not exist, a session_id will be generated,Then the generated session_id will be passed to the client as the COOKIE value
.
It is equivalent to executing the following COOKIE operation. Note that this step executes the setcookie() operation. COOKIE is sent in the header.
There is no output before this. PHP has another A function session_regenerate_id() If you use this function, there will be no output before this.               setcookie(session_name

(),

SESSON_ID ## () ##, # SESSION ##. ,//Default 0
## session.cookie_path ,
//Default '/' is valid in the current program and directory.cookie_domain
,//Default is empty                 ) D. If it exists then session_id =$_COOKIE[session_name];
　　　Then go to the folder specified by session.save_path and find the name 'SESS_'.session_id() file .
Read the contents of the file, deserialize it, and then put In $_SESSION

##　2、 is $_SESSION assignment　For example, add a new value
$_SESSION['test'] ='blah'; Then this$_SESSION will only be maintained in memory. When the script execution ends, Use to write the value of
$_SESSION to the folder specified by session_id , and then close the relevant resources. At this stage, it is possible to perform an operation to change the session_id, For example, destroy an old session_id and generate a brand new one session_id
.Half is used for custom session operations and role conversion, For example, Drupal
.Anonymous users of Drupal have A SESSION needs to use a new session_id
after logging in.

if (isset($_COOKIE[session_name()])) {
          　　setcookie(session_name(),'',time() -42000,'/');//旧session cookie过期
        　　}
        　　session_regenerate_id();//这一步会生成新的session_id
       　　//session_id()返回的是新的值

3, Write SESSION operation At the end of the script, the SESSION write operation will be performed, and the value in
$_SESSION will be written to the file named by session_id , may already exist, may need to create a new file.

## 4. Destroy SESSION The COOKIE sent by SESSION is generally an immediate COOKIE and is stored in memory. It will be deleted when the browser is closed. Will expire. If you need to force the expiration manually,
For example, log out instead of closing the browser, then you need to destroy the SESSION in the code. There are many methods,
        
1. setcookie(session_name(),session_id(),time() -8000000,..);//Execute before logging out
##              2. usset($_SESSION);//This will delete All $_SESSION data, after refreshing, COOKIE is passed, but there is no data. ##         
3. session_destroy();//This function is more thorough, delete $_SESSION, delete the session file, and session_id## When the browser is not closed, Refresh again, both 2 and 3 will have COOKIE transmitted, but the data cannot be found

2. User-defined session processing mechanism. php.ini configuration:

session.save_handler = user User-defined session The processing mechanism is more intuitive*session_set_save_handler('open','close','read','write','destroy ','gc');
1 .session_start(),
Execute open($save_path,$session_name)Open session operation handle
                                                                                                                  .save_handler = In the case of files, it is session.save_path, But if the user If it is determined, neither of these two parameters are used, and TRUE is returned directly to execute read($id
) to read the data

. //This parameter is automatically passed to session_id(), and can be operated through this value. ​ *
2.Script execution ends​ Execute write ($id,
$sess_data) //Two parameters, very Simple *3.假如用户需要session_destroy()
先执行destroy.在执行第2步

一个实际例子：

 //SESSION初始化的时候调用
      function open($save_path,$session_name)
      {
        global$sess_save_path;
        $sess_save_path=$save_path;
        return(true);
      }

      //关闭的时候调用
      function close()
      {
        return(true);
      }

      function read($id)
      {
        global$sess_save_path;
        $sess_file="$sess_save_path/sess_$id";
        return (string) @file_get_contents($sess_file);
      }
      //脚本执行结束之前，执行写入操作
      function write($id,$sess_data)
      {
        echo"sdfsf";
        global$sess_save_path;

        $sess_file="$sess_save_path/sess_$id";
        if ($fp= @fopen($sess_file,"w")) {
          $return=fwrite($fp,$sess_data);
          fclose($fp);
          return$return;
        } else {
          return(false);
        }

      }

      function destroy($id)
      {
        global$sess_save_path;

        $sess_file="$sess_save_path/sess_$id";
        return(@unlink($sess_file));
      }

      function gc($maxlifetime)
      {
        global$sess_save_path;

        foreach (glob("$sess_save_path/sess_*") as$filename) {
          if (filemtime($filename) +$maxlifetime<time()) {
            @unlink($filename);
          }
        }
        returntrue;
      }

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