Session management in PHP is the process of maintaining the state and data of a user session across multiple requests. Here are some key points to consider when working with PHP session management:

1. Starting a session: To start a session, you need to call the `session_start()` function at the beginning of your PHP script. This will initialize a new session or resume an existing session.

2. Storing session data: You can store data in the session using the `$_SESSION` superglobal array. For example, `$_SESSION[‘username’] = ‘john’;` will store the value ‘john’ in the session variable ‘username’. You can store any type of data in the session, including arrays and objects.

3. Retrieving session data: To retrieve session data, simply access the appropriate key in the `$_SESSION` superglobal. For example, `$username = $_SESSION[‘username’];` will assign the value stored in the session variable ‘username’ to the variable `$username`.

4. Destroying a session: To end a session and delete all session data, you can call the `session_destroy()` function. This will invalidate the current session and remove all session data from the server. However, you should note that this function does not unset the session variables or unset the session cookie, so you should also call `session_unset()` and `setcookie()` to properly clear the session data and cookie.

5. Session expiration: By default, PHP sessions expire after a certain period of inactivity (usually 24 minutes). You can modify the session expiration time by setting the `session.gc_maxlifetime` directive in the PHP configuration file. Additionally, you can manually set the expiration time for a session using the `session_set_cookie_params()` function.

6. Session security: To ensure session security, it is recommended to use SSL/TLS encryption to protect session data in transit. You should also regenerate the session ID after a user authenticates or changes their privilege level to minimize the risk of session fixation attacks.

7. Session hijacking prevention: You can prevent session hijacking attacks by binding the session to the client’s IP address or user agent. This can be done by setting the `session.use_strict_mode` and `session.use_only_cookies` directives in the PHP configuration file.

Overall, PHP provides a simple and flexible session management mechanism that allows you to store and retrieve data across multiple requests for maintaining session state. By following best practices and implementing appropriate security measures, you can ensure the integrity and security of your PHP sessions.