PHP session_start() 函数：为什么每次使用与 PHP 会话相关的任何东西时都需要它

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.

Answer 5

consider session_start() as your way of telling the php engine.... that you want to work with sessions.

and, as i understand it, always make that to be the first line ever in php page.

Answer 6

I was confused with the usage of session_start(); and every time I was using a session variable, I was calling session_start. Precisely, I had session_start(); more than once on each page (without even calling session_destroy()). For example,

// 1st call
session_start();

if (!isset($_SESSION['UserID']))    
{       
    // Do something    
}

else
{
   // Do something else
}

// .... some other code

// 2nd call
session_start();

if (!isset($_SESSION['UserID']))    
{       
    // Do something totally different   
}

else
{
   // Do something else totally different
}

This was creating a performance issue for me. So I ended up calling session_start(); just once at the very top of the page and everything seems to be working fine.

Answer 7

You have to call session_start once (and only once) in every file you want sessions to work in.

A common approach allowing you to only call it once is to have a dispatcher file as your index.php; call session_start in here and have this page include others based on the url's $_GET.

<?php
    session_start();
    if(isset($_GET['page']) && file_exists('pages/'.$_GET['page'].'.php') {
        include $_GET['page'];
    }
?>
//www.mysite.com/index.php?page=fish will display /pages/fish.php with session access