session_create_id

(PHP 7 >= 7.1.0, PHP 8)

session_create_id — Create new session id

Description

session_create_id(string $prefix = ""): string|false

session_create_id() is used to create new session id for the current session. It returns collision free session id.

If session is not active, collision check is omitted.

Session ID is created according to php.ini settings.

It is important to use the same user ID of your web server for GC task script. Otherwise, you may have permission problems especially with files save handler.

Parameters

prefix: If prefix is specified, new session id is prefixed by prefix. Not all characters are allowed within the session id. Characters in the range [a-zA-Z0-9,-] are allowed. Maximum length is 256 characters.

Return Values

session_create_id() returns new collision free session id for the current session. If it is used without active session, it omits collision check. On failure, false is returned.

Examples

Example #1 session_create_id() example with session_regenerate_id()

<?php
// My session start function support timestamp management
function my_session_start() {
    session_start();
    // Do not allow to use too old session ID
    if (!empty($_SESSION['deleted_time']) && $_SESSION['deleted_time'] < time() - 180) {
        session_destroy();
        session_start();
    }
}

// My session regenerate id function
function my_session_regenerate_id() {
    // Call session_create_id() while session is active to 
    // make sure collision free.
    if (session_status() != PHP_SESSION_ACTIVE) {
        session_start();
    }
    // WARNING: Never use confidential strings for prefix!
    $newid = session_create_id('myprefix-');
    // Set deleted timestamp. Session data must not be deleted immediately for reasons.
    $_SESSION['deleted_time'] = time();
    // Finish session
    session_commit();
    // Make sure to accept user defined session ID
    // NOTE: You must enable use_strict_mode for normal operations.
    ini_set('session.use_strict_mode', 0);
    // Set new custom session ID
    session_id($newid);
    // Start with custom session ID
    session_start();
}

// Make sure use_strict_mode is enabled.
// use_strict_mode is mandatory for security reasons.
ini_set('session.use_strict_mode', 1);
my_session_start();

// Session ID must be regenerated when
//  - User logged in
//  - User logged out
//  - Certain period has passed
my_session_regenerate_id();

// Write useful codes
?>

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User Contributed Notes 1 note

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rowan dot collins at gmail dot com ¶

7 years ago

This function is very hard to replicate precisely in userland code, because if a session is already started, it will attempt to detect collisions using the new "validate_sid" session handler callback, which did not exist in earlier PHP versions.

If the handler you are using implements the "create_sid" callback, collisions may be detected there. This is called when you use session_regenerate_id(), so you could use that to create a new session, note its ID, then switch back to the old session ID. If no session is started, or the current handler doesn't implement "create_sid" and "validate_sid", neither this function nor session_regenerate_id() will guarantee collision resistance anyway.

If you have a suitable definition of random_bytes (a library is available to provide this for versions right back to PHP 5.3), you can use the following to generate a session ID in the same format PHP 7.1 would use. $bits_per_character should be 4, 5, or 6, corresponding to the values of the session.hash_bits_per_character / session.sid_bits_per_character ini setting. You will then need to detect collisions manually, e.g. by opening the session and confirming that $_SESSION is empty.

<?php
function session_create_random_id($desired_output_length, $bits_per_character)
{
    $bytes_needed = ceil($desired_output_length * $bits_per_character / 8);
    $random_input_bytes = random_bytes($bytes_needed);
    
    // The below is translated from function bin_to_readable in the PHP source (ext/session/session.c)
    static $hexconvtab = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ,-';
    
    $out = '';
    
    $p = 0;
    $q = strlen($random_input_bytes);
    $w = 0;
    $have = 0;
    
    $mask = (1 << $bits_per_character) - 1;

    $chars_remaining = $desired_output_length;
    while ($chars_remaining--) {
        if ($have < $bits_per_character) {
            if ($p < $q) {
                $byte = ord( $random_input_bytes[$p++] );
                $w |= ($byte << $have);
                $have += 8;
            } else {
                // Should never happen. Input must be large enough.
                break;
            }
        }

        // consume $bits_per_character bits
        $out .= $hexconvtab[$w & $mask];
        $w >>= $bits_per_character;
        $have -= $bits_per_character;
    }

    return $out;
}
?>