In PHP, how can you destroy a single session variable and how can you destroy the entire session?

Introduction / Context:
Managing session lifecycle is vital in PHP applications, especially for security sensitive features such as user logins. Developers need to know how to remove individual session variables, for example when a temporary flag is no longer needed, and how to completely destroy a session when a user logs out. Interview questions about destroying sessions check whether developers understand both the PHP API and the difference between unsetting variables and ending the entire session.

Given Data / Assumptions:

• A PHP session has been started with session_start().
• Data is stored in the $_SESSION superglobal as key value pairs.
• Session data is stored on the server, and the session ID is stored on the client, usually in a cookie.
• The question asks about removing a particular variable and destroying the whole session.

Concept / Approach:
PHP sessions are represented by the $_SESSION array in the script and by a session file or other storage on the server. Removing one variable from the session usually means unsetting its entry in $_SESSION. Destroying the entire session involves clearing all session variables and removing the session data on the server, as well as invalidating the session ID. PHP provides functions such as unset(), session_unset(), and session_destroy() to handle these tasks. Correct use of these functions ensures that sensitive information is not left accessible after logout.

Step-by-Step Solution:
Step 1: To remove a single session variable, call unset() on the corresponding key in $_SESSION, for example unset($_SESSION["cart"]); this removes only that entry while leaving the session itself active. Step 2: To clear all session variables for the current session, you can call session_unset(), which empties the $_SESSION array for the running script. Step 3: To destroy the session data on the server and mark the session ID as no longer valid, call session_destroy() after session_start(). This deletes the session storage but does not automatically clear the superglobal or cookies. Step 4: A typical logout sequence is to call session_start(), then session_unset(), then session_destroy(), and optionally delete the session cookie by setting it with an expiry time in the past. Step 5: After these steps, any further requests with the old session ID will not resume the previous session because the data was removed on the server. Step 6: This method ensures both partial and complete removal of session data as required by application logic.

Verification / Alternative check:
You can test these behaviours in a simple script by starting a session, setting some $_SESSION values, and then unsetting one of them while printing the array. The removed key will no longer appear. In another script, call session_unset() and session_destroy() and observe that a subsequent request does not retain the previous session data. Checking the server session storage directory will show that the session file has been removed or invalidated.

Why Other Options Are Wrong:
Option b is wrong because deleting a cookie in the browser does not by itself remove server side session files, and restarting the server does not necessarily clear session storage unless configured specifically to do so. Option c is incorrect because exit() simply stops script execution; it does not automatically clean up sessions. Option d is wrong because renaming the script file does not affect existing sessions that may have been created by other requests or scripts.

Common Pitfalls:
A common pitfall is to call session_destroy() without first calling session_start(), which prevents the function from working as expected. Another issue is forgetting to invalidate the session cookie, which can lead to confusing behaviour on the client side. It is also important not to rely on partial clearing when a full logout is required; leaving sensitive keys inside $_SESSION can create security risks. Following the recommended sequence of unsetting specific keys, clearing all variables when needed, destroying the session, and expiring the cookie helps ensure that user sessions are properly terminated.

Final Answer:
To delete one session variable you unset the corresponding entry in $_SESSION, and to destroy the entire session you typically call session_unset() followed by session_destroy(), optionally deleting the session cookie as part of a full logout process.