Simple explanation of session expiration setting in php

This article is a detailed analysis and introduction to PHP's session expiration settings. Friends in need can refer to

Many people on the Internet have given answers: Modify php session.gc_maxlifetime in configuration file . If you want to know more about the session recycling mechanism, continue reading. (This article's environment is php5.2)

Overview: Every PHP request has a 1/100 probability (default value) of triggering "session recycling". If "session recycling" occurs, the files in /tmp/sess_* will be checked. If the last modification time exceeds 1440 seconds (the value of gc_maxlifetime), it will delete, which means these The session expires.

1. How does session exist on the server side (usually Apache with PHP module)?
By default, php will save the session in the /tmp directory, and the file name will be like this: sess_01aab840166fd1dc253e3b4a3f0b8381. Each file corresponds to a session.
more /tmp/sess_01aab840166fd1dc253e3b4a3f0b8381
username|s:9:”jiangfeng”;admin|s:1:”0″;
#Variable name|Type: length: value
Deleting the session file here means that the corresponding session is invalid.

2. How does session exist on the client side (usually the browser)?
session On the browser side, you only need to save the session ID (the unique ID generated by the server side). There are two ways to save: in cookie and in the url. If the session ID is saved in the cookie, you can see that there is a PHPSESID variable in the browser's cookie. If it is passed by URL, you can see the URL in the form:
index.php?PHPSESID=01aab840166fd1dc253e3b4a3f0b8381. (On the server side, use session.use_cookies to control which method is used)

3. On the server side, how does PHP determine whether the session file has expired?
If the "last modification time" to "now" exceeds gc_maxlifetime (default is 1440) seconds, this session file is considered expired. When the next session is recycled, if this file If it still has not been changed, the session file will be deleted (the session will expire).
Simply put, if I log in to a website and there is no operation within 1440 seconds (default value), then the corresponding session is considered to have expired.
So, modify the gc_maxlifetime variable in the php.ini file to extend the session expiration time: (for example, we modify the expiration time to 86400 seconds)
session.gc_maxlifetime = 86400
Then, restart you A web service (usually apache) will do.
Note: PHP5 uses a recycling mechanism when session expires. The time set here is 86400 seconds. If the session has not been modified within 86400 seconds, it will not be deleted until the next "recycling".

3. When does session "recycling" occur?
By default, for every PHP request, there will be a 1/100 probability of recycling, so it may be simply understood as "one recycling occurs for every 100 PHP requests." This probability is controlled by the following parameters
#The probability is gc_probability/gc_pisor
session.gc_probability = 1
session.gc_pisor = 100
Note 1:Assume that in this case gc_maxlifetime=120, if a session file was last modified 120 seconds ago, then the session will still be valid before the next recycling (1/100 probability) occurs.
Note 2: If your session uses session.save_path to save the session elsewhere, the session recycling mechanism may not automatically process expired session files. At this time, you need to delete expired sessions manually (or crontab) regularly: cd /path/to/sessions; find -cmin +24 | xargs rm

4. Some special cases
Because the recycling mechanism will check the "last modification time" of the file, if a session is active, but the session content is not If it has been changed, then the corresponding session file has not been changed. The recycling mechanism will think that this is a session that has not been active for a long time and delete it. This is something we don’t want to see. We can solve this problem by adding the following simple code:
60) $_SESSION['last_access'] = time(); ?>
The code will try to modify the session every 60 seconds.
Summary: If you want to modify the session expiration time, just modify the variable gc_maxlifetime. PHP5's session uses a passive recycling mechanism (garbage collection). Expired session files will not disappear by themselves, but expired sessions will be processed by triggering "recycling".

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