The execution timing of session_register_shutdown() was not considered before using exit()

In PHP development, the exit() function is a commonly used means to terminate script execution, especially when handling exceptions or directly returning results. However, many developers overlook a detail - after exit() is executed, some mechanisms of PHP may not execute as expected. This article focuses on a common misunderstanding: session_register_shutdown() (and similar registration shutdown callback function) execution timing when using exit() .

What is session_register_shutdown()?

The function registered by PHP through session_register_shutdown() is an "close callback function" that is automatically called after the script is executed. It is usually used to automatically complete the writing and closing of the session after the script is executed to ensure the integrity of the data.

 <?php
session_start();
session_register_shutdown();
$_SESSION['user'] = 'chatgpt';
exit();
?>

In the above code, session_register_shutdown() will be called after the script is executed, thereby safely writing and closing session.

Execution order of exit() and close callback functions

In most cases, exit() does not terminate all PHP mechanisms immediately. It stops the execution of subsequent code, but the registered shutdown function (including session_register_shutdown() ) will still be executed before the script lifecycle ends.

However, the "execution" order and some special cases here are very important:

• Normal process : exit() stops code execution, but the registered closing callback function will still be called to ensure that the session is closed normally.

• Exception termination or fatal error : Some cases, such as fatal errors, the closing function may not be executed.

• Early calls to exit() : If you call exit() before calling session_register_shutdown() , the closing function will not be registered and the session may not be closed normally.

Key reminder

Please be sure to ensure:

1. Registering a close function should be called earlier than exit() <br> Only by calling session_register_shutdown() first, the closing function will be executed normally after exit() .

2. Don't rely on exit() to write session immediately
   exit() does not prevent the shutdown function from running, but your code should not assume that the write is done immediately.

3. Avoid calling exit() in advance causing data loss <br> If the closing function is not registered before exit() is called, the session data will not be saved.

Actual cases

 <?php
session_start();

// The correct way to do it：Register to close the callback first
session_register_shutdown();

// operate session data
$_SESSION['user'] = 'm66.net';

// Terminate script
exit();

// The code here will not be executed，but session The closing function will be called automatically
?>

Counterexample:

 <?php
session_start();

// Exit directly，Close callback without registration
exit();

// session It may not be closed properly，data可能丢失
session_register_shutdown();
?>

Summarize

When stopping PHP script execution using exit() , make sure to register all closing callback functions first, especially session_register_shutdown() to ensure that session data can be written and closed correctly. A rational understanding of the PHP life cycle can help avoid data inconsistencies caused by abnormal program termination.

If you have further questions about the PHP life cycle or session mechanism, please continue to communicate.