Showing posts with label PHP PHPSESSID. Show all posts
Showing posts with label PHP PHPSESSID. Show all posts

Thursday, 4 September 2014

PHP Sessions

An alternative way to make data accessible across the various pages of an entire website is to use a PHP Session.
A session creates a file in a temporary directory on the server where registered session variables and their values are stored. This data will be available to all pages on the site during that visit.
The location of the temporary file is determined by a setting in the php.ini file called session.save_path. Bore using any session variable make sure you have setup this path.
When a session is started following things happen:
  • PHP first creates a unique identifier for that particular session which is a random string of 32 hexadecimal numbers such as 3c7foj34c3jj973hjkop2fc937e3443.
  • A cookie called PHPSESSID is automatically sent to the user's computer to store unique session identification string.
  • A file is automatically created on the server in the designated temporary directory and bears the name of the unique identifier prefixed by sess_ ie sess_3c7foj34c3jj973hjkop2fc937e3443.
When a PHP script wants to retrieve the value from a session variable, PHP automatically gets the unique session identifier string from the PHPSESSID cookie and then looks in its temporary directory for the file bearing that name and a validation can be done by comparing both values.
A session ends when the user loses the browser or after leaving the site, the server will terminate the session after a predetermined period of time, commonly 30 minutes duration.

Starting a PHP Session:

A PHP session is easily started by making a call to the session_start() function.This function first checks if a session is already started and if none is started then it starts one. It is recommended to put the call to session_start() at the beginning of the page.
Session variables are stored in associative array called $_SESSION[]. These variables can be accessed during lifetime of a session.
The following example starts a session then register a variable called counter that is incremented each time the page is visited during the session.
Make use of isset() function to check if session variable is already set or not.
Put this code in a test.php file and load this file many times to see the result:
<?php
   session_start();
   if( isset( $_SESSION['counter'] ) )
   {
      $_SESSION['counter'] += 1;
   }
   else
   {
      $_SESSION['counter'] = 1;
   }
   $msg = "You have visited this page ".  $_SESSION['counter'];
   $msg .= "in this session.";
?>
<html>
<head>
<title>Setting up a PHP session</title>
</head>
<body>
<?php  echo ( $msg ); ?>
</body>
</html>

Destroying a PHP Session:

A PHP session can be destroyed by session_destroy() function. This function does not need any argument and a single call can destroy all the session variables. If you want to destroy a single session variable then you can use unset() function to unset a session variable.
Here is the example to unset a single variable:
<?php
   unset($_SESSION['counter']);
?>
Here is the call which will destroy all the session variables:
<?php
   session_destroy();
?>

Turning on Auto Session:

You don't need to call start_session() function to start a session when a user visits your site if you can set session.auto_start variable to 1 in php.ini file.

Sessions without cookies:

There may be a case when a user does not allow to store cookies on their machine. So there is another method to send session ID to the browser.
Alternatively, you can use the constant SID which is defined if the session started. If the client did not send an appropriate session cookie, it has the form session_name=session_id. Otherwise, it expands to an empty string. Thus, you can embed it unconditionally into URLs.
The following example demonstrates how to register a variable, and how to link correctly to another page using SID.
<?php
   session_start();

   if (isset($_SESSION['counter'])) {
      $_SESSION['counter'] = 1;
   } else {
      $_SESSION['counter']++;
   }
?>
   $msg = "You have visited this page ".  $_SESSION['counter'];
   $msg .= "in this session.";
   echo ( $msg );
<p>
To continue  click following link <br />
<a  href="nextpage.php?<?php echo htmlspecialchars(SID); >">
</p>
The htmlspecialchars() may be used when printing the SID in order to prevent XSS related attacks.

PHP: Can sessions work without cookies?

If so, how does a session work without cookies enabled in PHP?

This is a great interview question because even if you do not know the answer, you could come up with a fairly accurate answer on your own with some basic knowledge of PHP sessions and some analytical thinking. See if you can possibly think of how PHP sessions would work without cookies enabled in the browser.

The answer to how PHP sessions can work without cookies

Sessions in PHP normally do use cookies to function. But, PHP sessions can also work without cookies in case cookies are disabled or rejected by the browser that the PHP server is trying to communicate with.

How PHP sessions work without cookies

PHP does two things in order to work without cookies:
1. For every HTML form that PHP finds in your HTML code (which of course can be part of a PHP file), PHP will automatically add a hidden input tag with the name PHPSESSID right after the <form> tag. The value of that hidden input tag would be whatever value PHP assigns your session ID. So, for example, the hidden input could look something like this: 
<form>

<input type="hidden" name="PHPSESSID" value="12345678" >

</form>

This way, when the form is submitted to the server, PHP will be able to retrieve the session identifier from the form and will know who it is communicating with on the other end, and will also know which session to associate the form parameters with if it is adding the form parameters to the PHP session.
2. PHP will find all the links in your HTML code, and will modify those links so that they have a GET parameter appended to the link itself. That GET parameter will also have the name of PHPSESSID, and the value will of course be the unique session identifier – so the PHP session ID will basically be a part of the URL query string.
So, for example, if your code has a link that originally looks like this: 
<a href="http://www.example.com">Go to this link><a/>
When modified by PHP to include the session ID, it could look something like this: 
<a href="http://www.example.com?PHPSESSID=72aa95axyz6cd67d82ba0f809277326dd">Go to this link</>

PHPSESSID can have it’s name changed in php ini file

Note that we said PHPSESSID is the name that will be used to hold the PHP session value. The name PHPSESSID can actually be changed to whatever you want if you modify the session.name value in the php.ini file.

What is a disadvantage of using PHP sessions without cookies enabled?

A disadvantage is that using PHP sessions without cookies is the fact that if you share a URL that has the PHP session ID appended to it with someone else, then they could potentially use the same exact session that you were using. It also opens you up to session hijacking – where a user’s session is deliberately stolen so that a hacker can impersonate you and do some damage.

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