Configuring Sessions and Save Handlers

This article deals with how to configure session management and fine tune it to your specific needs. This documentation covers save handlers, which store and retrieve session data, and configuring session behavior.

Save Handlers

The PHP session workflow has 6 possible operations that may occur. The normal session follows open, read, write and close, with the possibility of destroy and gc (garbage collection which will expire any old sessions: gc is called randomly according to PHP’s configuration and if called, it is invoked after the open operation). You can read more about this at php.net/session.customhandler

Native PHP Save Handlers

So-called native handlers, are save handlers which are either compiled into PHP or provided by PHP extensions, such as PHP-SQLite, PHP-Memcached and so on.

All native save handlers are internal to PHP and as such, have no public facing API. They must be configured by php.ini directives, usually session.save_path and potentially other driver specific directives. Specific details can be found in the docblock of the setOptions() method of each class. For instance, the one provided by the Memcached extension can be found on php.net.

While native save handlers can be activated by directly using ini_set('session.save_handler', $name);, Symfony provides a convenient way to activate these in the same way as it does for custom handlers.

Symfony provides drivers for the following native save handler as an example:

• NativeFileSessionHandler

Example usage:

use Symfony\Component\HttpFoundation\Session\Session;
use Symfony\Component\HttpFoundation\Session\Storage\Handler\NativeFileSessionHandler;
use Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage;

$sessionStorage = new NativeSessionStorage([], new NativeFileSessionHandler());
$session = new Session($sessionStorage);

Note

With the exception of the files handler which is built into PHP and always available, the availability of the other handlers depends on those PHP extensions being active at runtime.

Note

Native save handlers provide a quick solution to session storage, however, in complex systems where you need more control, custom save handlers may provide more freedom and flexibility. Symfony provides several implementations which you may further customize as required.

Custom Save Handlers

Custom handlers are those which completely replace PHP’s built-in session save handlers by providing six callback functions which PHP calls internally at various points in the session workflow.

The Symfony HttpFoundation component provides some by default and these can serve as examples if you wish to write your own.

• PdoSessionHandler
• MemcachedSessionHandler
• MigratingSessionHandler
• RedisSessionHandler
• MongoDbSessionHandler
• NullSessionHandler

Example usage:

use Symfony\Component\HttpFoundation\Session\Session;
use Symfony\Component\HttpFoundation\Session\Storage\Handler\PdoSessionHandler;
use Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage;

$pdo = new \PDO(...);
$sessionStorage = new NativeSessionStorage([], new PdoSessionHandler($pdo));
$session = new Session($sessionStorage);

Migrating Between Save Handlers

If your application changes the way sessions are stored, use the MigratingSessionHandler to migrate between old and new save handlers without losing session data.

This is the recommended migration workflow:

1. Switch to the migrating handler, with your new handler as the write-only one. The old handler behaves as usual and sessions get written to the new one:

   $sessionStorage = new MigratingSessionHandler($oldSessionStorage, $newSessionStorage);
   

2. After your session gc period, verify that the data in the new handler is correct.

3. Update the migrating handler to use the old handler as the write-only one, so the sessions will now be read from the new handler. This step allows easier rollbacks:

   $sessionStorage = new MigratingSessionHandler($newSessionStorage, $oldSessionStorage);
   

4. After verifying that the sessions in your application are working, switch from the migrating handler to the new handler.

Configuring PHP Sessions

The NativeSessionStorage can configure most of the php.ini configuration directives which are documented at php.net/session.configuration.

To configure these settings, pass the keys (omitting the initial session. part of the key) as a key-value array to the $options constructor argument. Or set them via the setOptions() method.

For the sake of clarity, some key options are explained in this documentation.

Session Cookie Lifetime

For security, session tokens are generally recommended to be sent as session cookies. You can configure the lifetime of session cookies by specifying the lifetime (in seconds) using the cookie_lifetime key in the constructor’s $options argument in NativeSessionStorage.

Setting a cookie_lifetime to 0 will cause the cookie to live only as long as the browser remains open. Generally, cookie_lifetime would be set to a relatively large number of days, weeks or months. It is not uncommon to set cookies for a year or more depending on the application.

Since session cookies are just a client-side token, they are less important in controlling the fine details of your security settings which ultimately can only be securely controlled from the server side.

Note

The cookie_lifetime setting is the number of seconds the cookie should live for, it is not a Unix timestamp. The resulting session cookie will be stamped with an expiry time of time() + cookie_lifetime where the time is taken from the server.

Configuring Garbage Collection

When a session opens, PHP will call the gc handler randomly according to the probability set by session.gc_probability / session.gc_divisor. For example if these were set to 5/100 respectively, it would mean a probability of 5%. Similarly, 3/4 would mean a 3 in 4 chance of being called, i.e. 75%.

If the garbage collection handler is invoked, PHP will pass the value stored in the php.ini directive session.gc_maxlifetime. The meaning in this context is that any stored session that was saved more than gc_maxlifetime ago should be deleted. This allows one to expire records based on idle time.

However, some operating systems (e.g. Debian) do their own session handling and set the session.gc_probability variable to 0 to stop PHP doing garbage collection. That’s why Symfony now overwrites this value to 1.

If you wish to use the original value set in your php.ini, add the following configuration:

# config/packages/framework.yaml
framework:
    session:
        gc_probability: null

You can configure these settings by passing gc_probability, gc_divisor and gc_maxlifetime in an array to the constructor of NativeSessionStorage or to the setOptions() method.

Session Lifetime

When a new session is created, meaning Symfony issues a new session cookie to the client, the cookie will be stamped with an expiry time. This is calculated by adding the PHP runtime configuration value in session.cookie_lifetime with the current server time.

Note

PHP will only issue a cookie once. The client is expected to store that cookie for the entire lifetime. A new cookie will only be issued when the session is destroyed, the browser cookie is deleted, or the session ID is regenerated using the migrate() or invalidate() methods of the Session class.

The initial cookie lifetime can be set by configuring NativeSessionStorage using the setOptions(['cookie_lifetime' => 1234]) method.

Note

A cookie lifetime of 0 means the cookie expires when the browser is closed.

Session Idle Time/Keep Alive

There are often circumstances where you may want to protect, or minimize unauthorized use of a session when a user steps away from their terminal while logged in by destroying the session after a certain period of idle time. For example, it is common for banking applications to log the user out after just 5 to 10 minutes of inactivity. Setting the cookie lifetime here is not appropriate because that can be manipulated by the client, so we must do the expiry on the server side. The easiest way is to implement this via garbage collection which runs reasonably frequently. The cookie_lifetime would be set to a relatively high value, and the garbage collection gc_maxlifetime would be set to destroy sessions at whatever the desired idle period is.

The other option is specifically check if a session has expired after the session is started. The session can be destroyed as required. This method of processing can allow the expiry of sessions to be integrated into the user experience, for example, by displaying a message.

Symfony records some basic metadata about each session to give you complete freedom in this area.

Session Cache Limiting

To avoid users seeing stale data, it’s common for session-enabled resources to be sent with headers that disable caching. For this purpose PHP Sessions has the sessions.cache_limiter option, which determines which headers, if any, will be sent with the response when the session in started.

Upon construction, NativeSessionStorage sets this global option to "" (send no headers) in case the developer wishes to use a Response object to manage response headers.

Caution

If you rely on PHP Sessions to manage HTTP caching, you must manually set the cache_limiter option in NativeSessionStorage to a non-empty value.

For example, you may set it to PHP’s default value during construction:

Example usage:

use Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage;

$options['cache_limiter'] = session_cache_limiter();
$sessionStorage = new NativeSessionStorage($options);

Session Metadata

Sessions are decorated with some basic metadata to enable fine control over the security settings. The session object has a getter for the metadata, getMetadataBag() which exposes an instance of MetadataBag:

$session->getMetadataBag()->getCreated();
$session->getMetadataBag()->getLastUsed();

Both methods return a Unix timestamp (relative to the server).

This metadata can be used to explicitly expire a session on access, e.g.:

$session->start();
if (time() - $session->getMetadataBag()->getLastUsed() > $maxIdleTime) {
    $session->invalidate();
    throw new SessionExpired(); // redirect to expired session page
}

It is also possible to tell what the cookie_lifetime was set to for a particular cookie by reading the getLifetime() method:

$session->getMetadataBag()->getLifetime();

The expiry time of the cookie can be determined by adding the created timestamp and the lifetime.