session_id

(PHP 4, PHP 5, PHP 7)

session_id — Get and/or set the current session id

Description

string session_id ([ string $id ] )

session_id() is used to get or set the session id for the current session.

The constant SID can also be used to retrieve the current name and session id as a string suitable for adding to URLs. See also Session handling.

Parameters

id: If id is specified, it will replace the current session id. session_id() needs to be called before session_start() for that purpose. Depending on the session handler, not all characters are allowed within the session id. For example, the file session handler only allows characters in the range a-z A-Z 0-9 , (comma) and - (minus)!

Note: When using session cookies, specifying an id for session_id() will always send a new cookie when session_start() is called, regardless if the current session id is identical to the one being set.

Return Values

session_id() returns the session id for the current session or the empty string ("") if there is no current session (no current session id exists).

User Contributed Notes

Riikka K

1 year ago


It may be good to note that PHP does not allow arbitrary session ids. The session id validation in PHP source is defined in ext/session/session.c in the function php_session_valid_key:

https://github.com/php/php-src/blob/master/ext/session/session.c

To put it short, a valid session id may consists of digits, letters A to Z (both upper and lower case), comma and dash. Described as a character class, it would be [-,a-zA-Z0-9]. A valid session id may have the length between 1 and 128 characters. To validate session ids, the easiest way to do it use a function like:

<?php

function session_valid_id($session_id)
{
    return preg_match('/^[-,a-zA-Z0-9]{1,128}$/', $session_id) > 0;
}

?>

session_id() itself will happily accept invalid session ids, but if you try to start a session using an invalid id, you will get the following error:

Warning: session_start(): The session id is too long or contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,'

gmillikan at t1shopper dot com

1 year ago


session_id() URL-decodes the session value.   For example let's say we use setcookie() to push a cookie down to a web browser.  When the browser makes the next page request the browser sends the cookie back up to us with headers like this:  Cookie: PHPSESSID=enGHumY%2C-2De-F-TDzNHVmE%2ChY5;

If we use session_id() to read the cookie it will output a value of this: enGHumY,-2De-F-TDzNHVmE,hY5 

The two values don't match!  Use either setrawcookie() or URL encode if you wish to match the original value.

Dario Gomes

4 years ago


Gosh, took a LOOONG time to figure this one out! If you have suhosin built into your PHP and can't get sessions to work after changing the session id through session_id(), try turning off suhosin's session encryption option in php.ini with:

suhosin.session.encrypt=Off

ab at ixo point ca

5 years ago


I was perplexed by inconsistent results with the session ID depending on whether I retrieve it using SID, COOKIE, or session_id().  I have found that session_id() is the most reliable method, whereas SID and COOKIE["PHPSESSIONID"] are sometimes undefined.

I used this simple script to quickly test the problem on my servers:

<?php
$a = session_id();
if(empty($a)) session_start();
echo "SID: ".SID."<br>session_id(): ".session_id()."<br>COOKIE: ".$_COOKIE["PHPSESSID"];
?>

Regardless of browser I see the COOKIE undefined on the first load and the other two defined, then SID is empty on subsequent reloads and COOKIE is defined, but session_id() is always defined.

If I insert the session_regenerate_id() method that jeff_zamrzla gives below the refresh the page, I get a new session_id() but the COOKIE value is initially the prior session_id() until I hit refresh a second time.  So again, session_id() proves to be the most reliable method.

It's probably not a bug since I found the behaviour to be consistent in PHP versions 5.2.14, 5.3.3 and 5.3.4, but I can't figure what I'm missing and hopefully this will help others who run into this.

Axel

7 years ago


The documentation for session_id is incomplete when it says: 
"For example, the file session handler only allows characters in the range a-z, A-Z and 0-9!".

It is untrue when changing the default for the session.hash_bits_per_character as Colin said. session_id may therefore contain "-" and ",".

http://fr.php.net/manual/en/session.configuration.php

infinito84 at gmail dot com

2 years ago


Get a shared session.

Sometimes is good can interchange messages and vars between one session and another, but PHP dont support this. I create this script that allows with session_id() change from current session to shared session (this is, info with scope to all sessions) for read and write info and back in to user session. The code: 

<?php
    ini_set('display_errors',1);
    error_reporting(E_ALL);
    
    function get_global($key){
        //Get current session
        if(session_status()!=PHP_SESSION_ACTIVE)session_start();
        $current_id=session_id();
        session_write_close();
        //Set a global session with session_id=1
        session_id(1);
        session_start();
        //Get superglobal value
        $value=null;
        if(isset($_SESSION[$key]))$value=$_SESSION[$key];
        session_write_close();
        //Set the before session
        session_id($current_id);
        session_start();
        return $value;
    }
    
    function set_global($key,$value){
        //Get current session
        if(session_status()!=PHP_SESSION_ACTIVE)session_start();
        $current_id=session_id();
        session_write_close();
        //Set a global session with session_id=1
        session_id(1);
        session_start();
        //Set superglobal value
        $_SESSION[$key]=$value;
        session_write_close();
        //Set the before session
        session_id($current_id);
        session_start();
    }
    //Example 
    //Begin my session normally
    session_start();
    if(empty($_SESSION['count'])){
        $_SESSION['count']=0;
        $_SESSION['color']="rgb(".rand(0,255).",".rand(0,255).",".rand(0,255).")";
    }
    $_SESSION['count']++;
    $id=session_id();
    //Get the superglobal
    $test=get_global("test");
    //Set the superglobal test with empty array if this dont set
    if($test==null)$test=array();
    //Get the superglobal
    $test=get_global("test");
    //Set values for each reload page and save the session_id that create it
    $test[]="<span style='color:".$_SESSION['color']."'>Value: ".rand(0,100)." SessionID: $id</span><br>";
    //Save the superglobal
    set_global("test",$test);
    //Show the superglobal
    foreach($test as $t){
        echo $t;
    }
    echo "<b>Reloads = ".$_SESSION['count'].", <span style='color:".$_SESSION['color']."'>This my color</span></b>";
    
    exit;
?>

cbarnes at bfinity dot net

10 years ago


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:



<?php

$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

session_regenerate_id();         

$ses_id = session_id();

 }

?>

simon at quo dot com dot au

10 years ago


Length of PHPSESSID appears to be 32 characters by default.

dmeweb at dibsplace dot com

5 years ago


If you look at the notes on cookies (set_cookie I think), you will see that you can not read a cookie on the page that it is set.  That is because the cookies are sent with the page request which comes, of course, before your PHP is run.  You have to wait until the next page request from the same source to read the cookie.

Colin

9 years ago


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.

Anonymous

9 years ago


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.

karlhaines at comcast dot net

12 years ago


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!

jpjounier at hotmail dot com

10 years ago


About the note from Cybertinus : 



The following test doesn't work, the code following is always executed :



<?php

if(!session_id())

{

// 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() == "")

{

session_start();

}

else

{

// Anything you want

}

?>

Anonymous

9 years ago


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:
http://www.php.net/manual/en/ref.session.php

Drugelis, Lietuva

5 years ago


I had a lot of trouble with session_regenerate_id() as it did not regenerate... Session_id() stayed the same no matter what (unless closing the window). I wanted to have different sid and empty vars for each session/page meeting a condition for security reasons.  Finally, this worked:

<?php
$a = session_id();
if ($a == '') session_start();
if ( ...add check if you want to regenerate and destroy vars on some condition only [recommended :)]...  ) 
 { session_unset(); //destroys variables
   session_destroy() //destroys session;
 }

$a = session_id();
if ($a == '') session_start();
if (!isset($_SESSION['safety']))
{
    session_regenerate_id(true);
    $_SESSION['safety'] = true;
}
$_SESSION['sessionid'] = session_id();
?>

Now you get different sid and session variables empty for each session_start if condition is met (i.e. user hits refresh on user/password form, which I needed badly :). Hope this helps someone out there.
Env: localhost
Note: condition is mandatory, otherwise it destroys on each load.

Igor Oliveira Ferreira

9 months ago


This can looks obvious, but as me, you can spend some hours to make a simple session work between different browsers and devices. These are the basics for me, but you can build upon.

<?php
if($_GET){
    //defining the session_id() before session_start() is the secret
    session_id($_GET['session_id']);
    session_start();
    echo "Data: " . $_SESSION['theVar'];
    //use your data before below commands
    session_destroy();
    session_commit();
}else{
    //common session statement goes here
    session_start();
    $session_id=session_id();
    $_SESSION['theVar'] = "theData";
    echo "your.php?session_id=" . $session_id;
}
?>

Andi, info at pragmaMx dot org

13 years ago


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

Francois

5 years ago


In php version 5.3.2 in my case each time a new session-id was generated after session_start() but all was working before correctly in previous versions. So I lost data from my current session (wrong session-id). There was always a $_POST or $_GET or $_COOKIE available with the session-name and session-id, so session_start() was taken this automatically. Now I have to execute session_id(..old id ..) before session_start() and a session is started for the same id.

Shiji Jiang

1 year ago


IMPORTANT NOTE:
If you assign a specific session ID to a user in your applet, then do not run the following code either while logout,
session_regenerate_id(TRUE);
USE:
session_regenerate_id();   instead.
OTHERWISE, setting the session id will no longer works for that user.

jeff_zamrzla

11 years ago


Try this code snippet, from a book by a security expert who says this is more secure to place on every page:



<?php

session_start();

$_SESSION['name'] = "YourSession"; 



if (!isset($_SESSION['initiated']))

{

    session_regenerate_id();

    $_SESSION['initiated'] = true;

}

?>

session_id

Description

Parameters

Return Values

See Also

User Contributed Notes