Configuring Sessions and Save Handlers

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

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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);

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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);

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.

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:

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# config.yml
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.

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.

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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:

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$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.:

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$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:

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

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

PHP 5.4 Compatibility

Since PHP 5.4.0, SessionHandler and SessionHandlerInterface are available. Symfony provides forward compatibility for the SessionHandlerInterface so it can be used under PHP 5.3. This greatly improves interoperability with other libraries.

SessionHandler is a special PHP internal class which exposes native save handlers to PHP user-space.

In order to provide a solution for those using PHP 5.4, Symfony has a special class called NativeSessionHandler which under PHP 5.4, extends from \SessionHandler and under PHP 5.3 is just a empty base class. This provides some interesting opportunities to leverage PHP 5.4 functionality if it is available.

Save Handler Proxy

A Save Handler Proxy is basically a wrapper around a Save Handler that was introduced to seamlessly support the migration from PHP 5.3 to PHP 5.4+. It further creates an extension point from where custom logic can be added that works independently of which handler is being wrapped inside.

There are two kinds of save handler class proxies which inherit from AbstractProxy: they are NativeProxy and SessionHandlerProxy.

NativeSessionStorage automatically injects storage handlers into a save handler proxy unless already wrapped by one.

NativeProxy is used automatically under PHP 5.3 when internal PHP save handlers are specified using the Native*SessionHandler classes, while SessionHandlerProxy will be used to wrap any custom save handlers, that implement SessionHandlerInterface.

From PHP 5.4 and above, all session handlers implement SessionHandlerInterface including Native*SessionHandler classes which inherit from SessionHandler.

The proxy mechanism allows you to get more deeply involved in session save handler classes. A proxy for example could be used to encrypt any session transaction without knowledge of the specific save handler.