PHP session validity session.gc_maxlifetime

A known effective method is to use session_set_save_handler to take over all session management work. Generally, the session information is stored in the database, so that all expired sessions can be deleted through SQL statements and the validity period of the session can be accurately controlled. This is also a commonly used method for large websites based on PHP. However, for ordinary small websites, it seems that there is no need to work so hard.
But the general Session has a limited lifespan. If the user closes the browser, the Session variables cannot be saved! So how can we achieve the permanent life span of Session?
As we all know, Session is stored on the server side. The user's file is obtained based on the SessionID provided by the client, and then the file is read to obtain the value of the variable. The SessionID can use the client's Cookie or the Query_String of the Http1.1 protocol (which is accessed (the part after the "?" in the URL) is transmitted to the server, and then the server reads the directory of the Session...
To realize the permanent life of the Session, you first need to understand the relevant settings of the Session in php.ini (open the php.ini file , in the "[Session]" section):
1. session.use_cookies: The default value is "1", which means that SessionID is passed by Cookie, otherwise it is passed by Query_String;
2. session.name: This is where SessionID is stored The variable name may be Cookie or Query_String to pass. The default value is "PHPSESSID";
3. session.cookie_lifetime: This represents the time the SessionID is stored in the client cookie. The default is 0, which means that the browser closes the SessionID. It's invalid... It's because of this that the Session cannot be used permanently!
4. session.gc_maxlifetime: This is the time that Session data is stored on the server side. If this time is exceeded, the Session data will be automatically deleted!
There are many more settings, but these are the ones related to this article. Let’s start with the principles and steps of using permanent Session.
As mentioned before, the server reads Session data through SessionID, but generally the SessionID sent by the browser is gone after the browser is closed, then we only need to manually set the SessionID and save it, no...
If You have the operating authority of the server, so setting this is very, very simple. You just need to perform the following steps:
1. Set "session.use_cookies" to 1 and turn on Cookie to store SessionID, but the default is 1, generally no need to modify;
2. Change "session.cookie_lifetime" to positive infinity (of course there is no parameter for positive infinity, but there is no difference between 999999999 and positive infinity);
3. Set "session.gc_maxlifetime" to the same time as "session.cookie_lifetime" ;
It is clearly stated in the PHP documentation that the parameter for setting the session validity period is session.gc_maxlifetime. This parameter can be modified in the php.ini file or through the ini_set() function. The problem is that after many tests, modifying this parameter basically has no effect, and the session validity period still maintains the default value of 24 minutes.
Due to the working mechanism of PHP, it does not have a daemon thread to scan session information regularly and determine whether it is invalid. When a valid request occurs, PHP will decide whether to start a GC (Garbage Collector) based on the value of the global variable session.gc_probability/session.gc_divisor (which can also be modified through the php.ini or ini_set() function).
By default, session.gc_probability = 1, session.gc_divisor = 100, which means there is a 1% probability that GC will be started. The job of the GC is to scan all session information, subtract the last modification time (modified date) of the session from the current time, and compare it with the session.gc_maxlifetime parameter. If the survival time has exceeded gc_maxlifetime, delete the session.
So far, everything is working normally. So why does gc_maxlifetime become invalid?
By default, session information will be saved in the system’s temporary file directory in the form of text files. Under Linux, this path is usually tmp, and under Windows, it is usually C:WindowsTemp. When there are multiple PHP applications on the server, they will save their session files in the same directory. Similarly, these PHP applications will also start GC at a certain probability and scan all session files.
The problem is that when GC is working, it does not distinguish between sessions on different sites. For example, site A's gc_maxlifetime is set to 2 hours, and site B's gc_maxlifetime is set to the default 24 minutes. When the GC of site B starts, it will scan the public temporary file directory and delete all session files older than 24 minutes, regardless of whether they come from site A or B. In this way, the gc_maxlifetime setting of site A is useless.
Once you find the problem, it’s easy to solve it. Modify the session.save_path parameter, or use the session_save_path() function to point the directory where the session is saved to a dedicated directory. The gc_maxlifetime parameter works normally.
Strictly speaking, is this a bug in PHP?
Another problem is that gc_maxlifetime can only guarantee the shortest time for the session to survive, and cannot be saved. After this time, the session information will be deleted immediately. Because GC is started based on probability and may not be started for a long period of time, a large number of sessions will still be valid after exceeding gc_maxlifetime.
One way to solve this problem is to increase the probability of session.gc_probability/session.gc_divisor. If mentioned to 100%, this problem will be completely solved, but it will obviously have a serious impact on performance. Another method is to determine the lifetime of the current session in your code. If it exceeds gc_maxlifetime, clear the current session.
But if you do not have the operating authority of the server, it will be more troublesome. You need to rewrite the SessionID through the PHP program to achieve permanent session data storage. Check the function manual of php.net and you can see the "session_id" function: if no parameters are set, the current SessionID will be returned. If the parameters are set, the current SessionID will be set to the given value...
As long as you use a permanent cookie and add the "session_id" function, you can save permanent session data!
But for convenience, we need to know the "session.name" of the server settings, but most users do not have permission to view the server's php.ini settings. However, PHP provides a very good function "phpinfo", which can be used to view almost all settings. PHP information!
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b2386ffb911b14667cb8f0f91ea547a7PHP related information display6e916e0f7d1e588d4f442bf645aedb2f
61bbb8f2777cc17130c4b8e7bcd1a875
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Open the editor and enter If you run the above code in the browser, you will see PHP related information (as shown in Figure 1). There is a "session.name" parameter, which is the server "session.name" we need, usually "PHPSESSID".
After writing down the name of the SessionID, we can achieve permanent session data storage!
The code is as follows:

session_start(); 
ini_set('session.save_path','/tmp/'); 
//6个钟头 
ini_set('session.gc_maxlifetime',21600); 
//保存一天 
$lifeTime = 24 * 3600; 
setcookie(session_name(), session_id(), time() + $lifeTime, "/");

Postscript:
In fact, true permanent storage is impossible, because the storage time of cookies is limited, and the space of the server is also limited... But for some sites that need to be saved for a long time, the above method is enough enough!
Put the session into mysql Example:
Create a table in the database: session (sesskey varchar32, expiry int11, value longtext)
code:
The database has been connected before the code is executed.
The code is as follows:

define('STORE_SESSIONS','mysql'); 
if (STORE_SESSIONS == 'mysql') { 
if (!$SESS_LIFE = get_cfg_var('session.gc_maxlifetime')) { 
$SESS_LIFE = 1440; 
} 
function _sess_open($save_path, $session_name) { 
// 如果没有连接数据库，可以在此执行mysql_pconnect,mysql_select_db 
return true; 
} 
function _sess_close() { 
return true; 
} 
function _sess_read($key) { 
$value_query = mysql_query("select value from sessions where sesskey = '" .addslashes($key) . "' and expiry > '" . time() . "'"); 
$value = mysql_fetch_array($value_query); 
if (isset($value['value'])) { 
return $value['value']; 
} 
return false; 
} 
function _sess_write($key, $val) { 
global $SESS_LIFE; 
$expiry = time() + $SESS_LIFE; 
$value = $val; 
$check_query = mysql_query("select count(*) as total from sessions where sesskey = '" . addslashes($key) . "'"); 
$check = mysql_fetch_array($check_query); 
if ($check['total'] > 0) { 
return mysql_query("update sessions set expiry = '" . addslashes($expiry) . "', value = '" . addslashes($value) . "' where sesskey = '" . addslashes($key) . "'"); 
} else { 
return mysql_query("insert into sessions values ('" . addslashes($key) . "', '" . addslashes($expiry) . "', '" . addslashes($value) . "')"); 
} 
} 
function _sess_destroy($key) { 
return mysql_query("delete from sessions where sesskey = '" . addslashes($key) . "'"); 
} 
function _sess_gc($maxlifetime) { 
mysql_query("delete from sessions where expiry < &#39;" . time() . "&#39;"); 
return true; 
} 
session_set_save_handler(&#39;_sess_open&#39;, &#39;_sess_close&#39;, &#39;_sess_read&#39;, &#39;_sess_write&#39;, &#39;_sess_destroy&#39;, &#39;_sess_gc&#39;); 
} 
danoo_session_name( &#39;dtvSid&#39; ); 
danoo_session_save_path(SESSION_WRITE_DIRECTORY);

I still don’t understand where the open and write parameters come from.
Two commonly used functions to modify php.ini configuration:
get_cfg_var('session.gc_maxlifetime'): Get the value of session.gc_maxlifetime
ini_set('session.cookie_lifetime','0'): Set the value of session.cookie_lifetime to 0 .

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