As the life-cycle of a session handler is fairly complex, I found it difficult to understand when explained using just words - so I traced the function calls made to a custom SessionHandler, and created this overview of precisely what happens when you call various session methods:

https://gist.github.com/mindplay-dk/623bdd50c1b4c0553cd3

I hope this makes it considerably easier to implement a custom SessionHandler and get it right the first time :-)

Those who are planning to implement your own SessionHandler, let's say using a Database system, please make sure your 'create_sid' method creates a new record in your database with empty string '' as your 'data' using the new session ID created, otherwise when 'read' method is called (which is always called no matter if it is a brand new session or not), you will get an error because there is no record with that session ID yet.  The funny part is that the error you get will sound like PHP is trying open the session on your local drive (coming from your .ini file).

Your custom session handler should not contain calls to any of the session functions, such as session_name() or session_id(), as the relevant values are passed as arguments on various handler methods. Attempting to obtain values from alternative sources may not work as expected.

The best way to set up your own session handler is to extend the native SessionHandler (override the 7 methods + constructor as per your need, keeping the same signatures).  Optionally, you can also implement SessionUpdateTimestampHandlerInterface if you plan to use 'lazy_write':

Option 1:

class MyOwnSessionHandler extends \SessionHandler { ..... }

Option 2:

class MyOwnSessionHandler extends \SessionHandler implements \SessionUpdateTimestampHandlerInterface { ..... }

I would NOT recommend to do this:

class MyOwnSessionHandler implements \SessionHandlerInterface, \SessionIdInterface, \SessionUpdateTimestampHandlerInterface { ... }

If you are curious, here are the methods called in the order (using PHP 8.2 with XAMPP v3.3.0 on Windows 11 64-bit):

- open (always called)

- validateId and/or create_sid:

    validateId is called if you implement SessionUpdateTimestampHandlerInterface.  If validation fails, create_sid is called.     

    create_sid is called if a new session ID is needed: new session, etc.

- read (always called)

- write OR updateTimestamp OR destroy:

    if you call 'destroy', neither 'write' or 'updateTimestamp' is called,

    if you have the 'lazy_write' on AND implemented SessionUpdateTimestampHandlerInterface, 
    then 'updateTimestamp' is called instead of 'write' if nothing changes.

- close (always called)

I made this gist to provide a complete overview of the PHP session handler life cycle updated to version 7.0 or above. In particular, I wanted to emphasize what methods and in what order are called when the native PHP functions are used for session management.

https://gist.github.com/franksacco/d6e943c41189f8ee306c182bf8f07654

I hope this analysis will help all the developers interested in understanding in detail the native session management performed by PHP and what a custom session handler should do.
Any comment or suggestion is appreciated.

Be aware that when you inherit from \SessionHandler you implicitly disable session strict mode because it doesn't implement SessionUpdateTimestampHandlerInterface (see: https://www.php.net/manual/en/session.configuration.php#ini.session.use-strict-mode) thus validateId() is not called.

It's CRUCIAL to call the session_regenerate_id() function whenever any privilege level is changed (i.e. during authentication, password/permission/user role change) to mitigate the session fixation attack!

Here is a wrapper to log in a file each session's operations. Useful to investigate sessions locks (which prevent PHP to serve simultaneous requests for a same client).
Just change the file name at the end to dump logs where you want. 

class DumpSessionHandler extends SessionHandler {
    private $fich;

    public function __construct($fich) {
        $this->fich = $fich;
    }

    public function close() {
        $this->log('close');
        return parent::close();
    }

    public function create_sid() {
        $this->log('create_sid');
        return parent::create_sid();
    }

    public function destroy($session_id) {
        $this->log('destroy('.$session_id.')');
        return parent::destroy($session_id);
    }

    public function gc($maxlifetime) {
        $this->log('close('.$maxlifetime.')');
        return parent::gc($maxlifetime);
    }

    public function open($save_path, $session_name) {
        $this->log('open('.$save_path.', '.$session_name.')');
        return parent::open($save_path, $session_name);
    }

    public function read($session_id) {
        $this->log('read('.$session_id.')');
        return parent::read($session_id);
    }

    public function write($session_id, $session_data) {
        $this->log('write('.$session_id.', '.$session_data.')');
        return parent::write($session_id, $session_data);
    }

    private function log($action) {
        $base_uri = explode('?', $_SERVER['REQUEST_URI'], 2)[0];
        $hdl = fopen($this->fich, 'a');
        fwrite($hdl, date('Y-m-d h:i:s').' '.$base_uri.' : '.$action."\n");
        fclose($hdl);
    }
}
ini_set('session.save_handler', 'files');
$handler = new DumpSessionHandler('/path/to/dump_sessions.log');
session_set_save_handler($handler, true);

php -S localhost:8000 -t foo/

touch index.php

vi index.php
============================================================
class NativeSessionHandler extends \SessionHandler
{
    public function __construct($savePath = null)
    {
        if (null === $savePath) {
            $savePath = ini_get('session.save_path');
        }

        $baseDir = $savePath;

        if ($count = substr_count($savePath, ';')) {
            if ($count > 2) {
                throw new \InvalidArgumentException(sprintf('Invalid argument $savePath \'%s\'', $savePath));
            }

            // characters after last ';' are the path
            $baseDir = ltrim(strrchr($savePath, ';'), ';');
        }

        if ($baseDir && !is_dir($baseDir) && !@mkdir($baseDir, 0777, true) && !is_dir($baseDir)) {
            throw new \RuntimeException(sprintf('Session Storage was not able to create directory "%s"', $baseDir));
        }

        ini_set('session.save_path', $savePath);
        ini_set('session.save_handler', 'files');
    }
}

$handler = new NativeSessionHandler("/var/www/foo");
session_set_save_handler($handler, true);
session_start();
$a = $handler->write("aaa","bbbb");var_dump($a);exit;

============================================================

output：bool(false)