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PHP : Function Reference : Session Handling Functions : session_id


Get and/or set the current session id (PHP 4, PHP 5)
string session_id ( [string id] )

$ses_id session_id();

// create a new session ID and start a new
$ses_id session_id();


Related Examples ( Source code ) » session_id

Code Examples / Notes » session_id

andi, info

you can also add the iframe tag:
ini_set("url_rewriter.tags", "a=href,area=href,frame=src,iframe=src,input=src,form=fakeentry");


Try this code snippet, from a book by a security expert who says this is more secure to place on every page:
$_SESSION['name'] = "YourSession";
if (!isset($_SESSION['initiated']))
   $_SESSION['initiated'] = true;


The higher you set session.hash_bits_per_character the shorter your session_id will become by using more bits per character. The possible values are 4, 5, or 6.
When using sha-1 for hashing (by setting ini_set('session.hash_function', 1) the following session string lengths are produced by the three session.hash_bits_per_character settings:
4 - 40 character string
5 - 32 character string
6 - 27 character string
It would seem desirable to use sha-l with 5 bits_per_character because this will emulate a standard 32 character md5 string and make a would-be attacker think that is what you're hashing with.


Saving sessions to file and generating the ID's with Sha1 and 6 bit/character on Windows machines gives a higher chance of session collision (the filesystem is not case sensitive)


Rewriting URL's is not suggested for obvious security issues. Please be careful with register_globals when using sessions! Check that all information you recieve from a user is valid before accepting it!

25-mar-2007 08:06

Regarding Colin's comment, note that setting hash_bits_per_character to 5 results in characters ranging from 0-9 and a-v. Most attackers would be wise enough to realize what was going on when they saw a letter in g-v.  The probability of not seeing a letter in g-v is somewhere around 2^-32.


Note that Firefox and Mozilla use the same process for launching new windows or tabs, they will pick up the same session id as the previous windows until the parent process dies or is closed. This may cause undesired results if the session id is stored in a db and checked, a solution is to check at the new entry point (new tab or window if the user went back to the index page) for an existing session. If a session id exists and a new one is required use something like:
$ses_id = session_id();
$bsid_exists = false;
$bsid_exists = check_session_id_from_db($ses_id);
if ($bsid_exists){
//This is a reentry and the session already exists
// create a new session ID and start a new
$ses_id = session_id();


Length of PHPSESSID appears to be 32 characters by default.


Killing the session_id when using cookies to store the session_id. Useful when needing to recreate a user with different session information during an open session.

22-aug-2006 09:15

In response to simon at quo dot com dot au:
The PHPSESSID is produced using an hash function. By default, it uses MD5 which produces 128 bits long (i.e: 16 bytes long) hashes.
But, since some bytes' values may not be used in the HTTP header, PHP outputs the hash in its hexadecimal representation, thus resulting in a 32 bytes long text.
Starting with  PHP 5.0, you can change the hash function used (by setting "session.hash_function" to whatever function you want to use in php.ini).
You may for example set it to 1 to switch to SHA-1 which produces 160 bits (20 bytes) long hashes.
Please also note that another setting was introduced in PHP 5 (session.hash_bits_per_character) which sort of "compresses" the hash. Thus, resulting in what seems to be a shorter hash.
This feature helps you improve your application's security by producing IDs that are harder to prodict for a malicious attacker.
More information on those settings is provided on:


in addition to what said "jpjounier at hotmail dot com" it's EXTREMELY important to test if "session_id()" gives a result or not.
If you do not, you take the risk to have a new session starting over with the loss of your session variables.
php.ini can be different on each webserver you'll host your script on.
If you only rely on the behavior of your script on one server, you might have trouble on others.
One VERY important session's option of php.ini is 'session.auto_start' . If setted at 'true', session will then start automaticaly for each page that user will open without the need of using session_start().
The point is that if you have a script like this one:
   $login = $_POST['login'];
   $pass = $_POST['pass'];
   if($user->connection($login,$pass){ // user logging validation
       session_start();     //start the session
       $_SESSION['user_logged'] = true;  // user logged in
       header('location : control_panel.php');  // go to control panel
   else {  // go back to logging page
       header('location : logging.php?' . $user->error_string);
   echo $_SESSION['user_logged'];
All will be ok as long as 'session.auto_start ' is at 'false'. Otherwise, you'll have a brand new session created with empty $_SESSION and new SID.
This has been brain breaker for me until I realise that my webserver has 'session.auto_start' at 'true' and that I couldn't change it (some webhost doesn't let user change php.ini options).
So I changed the code this way:
   if (session_id() == "") session_start(); // if no active session we start a new one
   echo $_SESSION['user_logged'];
And then everything is ok regardless if 'session.auto_start' is set to 'false' or 'true'.
Furthermore, as said 'risaac at deadletter dot com' (04-Apr-2006 08:46) in a comment on 'session_write_close()', it might be a clever thing to check if the newly activated session has $_SESSION empty or setted with the value you passed by as expected. If empty, then you might have launched a brand new session and surely lost every fromer data unless you know what was the SID (passed by $_GET for example, not recommended but possible).


I was using session_id but i needed to send more than one variable across to another php page. I was trying to create a link to an xml file and i needed to send the name of the file to another php document to open the file using DOM or SimpleXML.
I sent the variable across in the hyperlink and gathered the variable using the $_POST['$VarName'] command.
This is the code i used:
echo"\t\t<td><a href=\"showArticle.php?file=".$title."\" name=\"$title\">View Article</a></td>\n";
$file = $_GET["file"];

eduardo dot sandino

Hi i have some problems with the session id it get lost after a postback, and the session.auto_start is = 0 i don´t know whay it goes to this...
I don´t if it cos i am debuging my application with Eclipse PDT + xDebug + Apache and something is missing or is just something i didn´t see
this is my code..
$sesion_id = session_id();//$_COOKIE["PHPSESSID"];
catch(Exception $e){
if( $debug_habilitado ){
could be the PHPSESSID key something to do ¿???


About the note from Cybertinus :
The following test doesn't work, the code following is always executed :
// Always executed even if there's already an opened session
session_id() returns an empty string if there is no current session, so to test if a session already exists, it's better to write this :
if(session_id() == "")
// Anything you want


$_POST is not a command, it's kind of a PHP-Global variable.
You use get here ($_GET) not POST

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