Understand and analyze the PHP session mechanism from the manual

Understand and analyze the PHP session mechanism from the manual_PHP Tutorial

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Release： 2016-07-21 15:26:21

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session.save_handler = files
1. session_start()

session_start() is the beginning of the session mechanism. It has a certain probability of starting garbage collection because the session is stored in a file. PHP's own garbage collection is invalid. SESSION recycling requires deleting files. This probability is determined by the configuration of php.ini, but some systems have session.gc_probability = 0, which means the probability is 0, and Garbage collection is implemented through cron scripts.

Copy code The code is as follows:

 
session.gc_probability = 1 
session.gc_divisor = 1000 
session. gc_maxlifetime = 1440//The default expiration time is 24 minutes
//The probability is session.gc_probability/session.gc_divisor. The result is 1/1000, 
//It is not recommended to set it too small, because the garbage collection of the session needs to check every Whether the 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. Need Write your own script 

session will determine whether there is currently $_COOKIE[session_name()]; session_name() returns the COOKIE key value that saves session_id. This value can be found from php.ini session.name = PHPSESSID //Default value PHPSESSID

If it does not exist, a session_id will be generated, and then the generated session_id will be passed to the client as the value of COOKIE. It is equivalent to executing the following COOKIE operation. Note that this step executes setcookie () operation, COOKIE is sent in the header, and there is no output before this. PHP has another function session_regenerate_id(). If you use this function, there is no output before this.

Copy code The code is as follows:

 
setcookie(session_name(), 
session_id(), 
session. cookie_lifetime, //default 0 
session.cookie_path, //default '/' is valid in the current program and directory 
session.cookie_domain, //default empty 
) 

If it exists, then session_id = $_COOKIE[session_name]; Then go to the folder specified by session.save_path to find the file named 'SESS_' . session_id(). Read the contents of the file, deserialize it, and put it in $_SESSION.

2. Assign a value to $_SESSION

For example, add a new value $_SESSION['test'] = 'blah'; then this $_SESSION will only be maintained in memory , when the script execution ends, write the value of $_SESSION to the folder specified by session_id, and then close the related resources.

At this stage, it is possible to perform operations to change the session_id, such as destroying an old session_id and generating a new session_id. Half of it is used for custom session operations and role conversion, such as Drupal. Drupal anonymous users There is a SESSION, and it needs to use a new session_id after logging in.

Copy code The code is as follows:

 
if (isset($_COOKIE[session_name()])) { 
setcookie (session_name(), '', time() - 42000, '/');//Old session cookie expires
} 
session_regenerate_id();//This step will generate a new session_id 
// session_id() returns the new value 

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 session_id, which may already exist , new files may need to be created.

4. Destroy SESSION

The COOKIE sent by SESSION is generally an instant COOKIE and is stored in memory. It will expire when the browser is closed. If artificial intelligence is required To force expiration, such as logging out instead of closing the browser, you need to destroy the SESSION in the code. There are many methods:

setcookie(session_name(), session_id(), time() - 8000000, .. );//Execute
usset($_SESSION);//This will delete all $_SESSION data before logging out. After refreshing, COOKIE is passed, but there is no data.
session_destroy();//This function is more thorough, delete $_SESSION, delete the session file, and session_id
When you do not close the browser, refresh again, 2 and 3 will have COOKIE passed, but cannot be found to data.

session.save_handler = user
User-defined session processing mechanism, more intuitive session_set_save_handler('open', 'close', 'read', 'write', 'destroy', 'gc');

session_start(), execute open($save_path, $session_name) to open the session operation handle. $save_path is session.save_path when session.save_handler = files, but if the user customizes it, neither of these two parameters will be used, and TRUE will be returned directly. Execute read($id) to read data from it. //This parameter is automatically passed to session_id(), and you can operate through this value.

The script execution ends, execute write($id, $sess_data) //Two parameters, very simple

If the user needs session_destroy(), execute destroy first, and then execute step 2

A practical example:

Copy code The code is as follows:

 
//Called when SESSION is initialized
function open($save_path, $session_name) 
{ 
global $sess_save_path; 
$sess_save_path = $save_path; 
return(true); 
} 

// When closing, call 
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); 
} 
//Before the script execution ends, perform the write operation
 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); 
} 
} 
return true; 
} 