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In PHP, the session_register_shutdown function and the session.auto_start configuration item are important components of the management session behavior. Understanding their mutual influence will help better control the life cycle of the session and improve the stability and performance of the application.
session_register_shutdown() is a mechanism used within PHP to register session shutdown functions. It is automatically called at the end of script execution and is used to ensure that session data can be written and closed correctly. This function is usually automatically called by the PHP session module, and the developer does not need manual intervention.
Its function is to register a callback function for session writing and closing, ensuring that session data is not lost at the end of the script.
session.auto_start is a boolean configuration item in php.ini that controls whether the session is automatically started at the beginning of each request. If set to 1 , PHP will automatically call session_start() at the beginning of the request, thereby initializing the session; if set to 0 , the developer needs to call session_start() manually.
Turning on session.auto_start means:
Automatically initialize sessions for each request
Developers do not need to explicitly call session_start()
Automatically load and write session data
When session.auto_start=1 , PHP will automatically call session_start() , and session_start() will call session_register_shutdown() to register the shutdown function. This ensures that the session write and close operations are performed regardless of how the script exits.
If session.auto_start=0 and the developer calls session_start() manually, session_register_shutdown() will also be called.
If the developer repeatedly calls session_start() in the script, or uses session_register_shutdown() incorrectly, it may cause the callback function to register exception, with the following problems:
Session data is not written in time, resulting in loss or inconsistency
Session lock is not released, blocking subsequent requests
When the script exception ends, the closing function is not called, causing resource leakage
In addition, if the developer manually closes and restarts the session when session.auto_start=1 , multiple calls to session_register_shutdown() will be triggered, which may cause callback function conflicts.
When session.auto_start is enabled, avoid explicit calls to session_start() in the code to prevent repeated registration of closing callbacks.
If you close session.auto_start , make sure session_start() is called before accessing session data.
Avoid calling session_register_shutdown() directly, which is automatically managed by PHP internally.
After processing the session, use session_write_close() to explicitly write and release the lock.
<?php
// Assumptionssession.auto_start=1，The following code does not need to be calledsession_start()
echo "Automatically start the session，currentsession_id：" . session_id();
// Access or set session data
$_SESSION['user'] = 'Zhang San';
// At the end of the script，session_register_shutdownAutomatic call，Will writesessiondata
?>
<?php
// Assumptionssession.auto_start=0，Start the session manually
session_start();
echo "Start the session manually，currentsession_id：" . session_id();
$_SESSION['user'] = 'Li Si';
session_write_close(); // 提前写入会话data并释放锁
?>
The official PHP manual introduces the session mechanism in detail:
