PHP session_start() function: Why I need it everytime I use anything related to PHP sessions

Question

For logging out a user from my website, I am redirecting the page to logout.php where I am using session_destroy() function. Even there also, logout functionality is not working without session_start() function. By adding session_start() function before session_destroy() function, I am able to logout the user successfully.

Why do I need to use session_start() function everytime and in every page where I am doing something related to sessions?

Answer 1

session_destroy() destroys the active session. If you do not initialized the session, there will be nothing to be destroyed.

Answer 2

Why do I need to use session_start() function everytime and in every page where I am doing something related to sessions?

So PHP knows which session to destroy. session_start() looks whether a session cookie or ID is present. Only with that information can you destroy it.

Answer 3

In the default configuration, PHP Sessions operate off of the hard disk. PHP asks you to explicitly tell it when you need this support to avoid unnecessary disk IO.

session_start() also tells PHP to find out if the user's session exists.

Answer 4

session_start() creates a session or resumes the current one based on a session identifier passed via a GET or POST request, or passed via a cookie.

as per http://php.net/manual/en/function.session-start.php

Essentially by calling session_start(), PHP reads the header and cross references that session ID to what is on your system(file system/database/etc), which can then populate the $_SESSION that is relavent to that specific user. Which in turn allows you to call session_destroy() because it knows what session to actually destroy.