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Sessions and Cookies

Sessions and cookies allow data to be persisted across multiple user requests. In plain PHP you may access them through the global variables $_SESSION and $_COOKIE, respectively. Yii encapsulates sessions and cookies as objects and thus allows you to access them in an object-oriented fashion with additional useful enhancements.

Sessions

Like requests and responses, you can get access to sessions via the session application component which is an instance of [[yii\web\Session]], by default.

Opening and Closing Sessions

To open and close a session, you can do the following:

$session = Yii::$app->session;

// check if a session is already open
if ($session->isActive) ...

// open a session
$session->open();

// close a session
$session->close();

// destroys all data registered to a session.
$session->destroy();

You can call [[yii\web\Session::open()|open()]] and [[yii\web\Session::close()|close()]] multiple times without causing errors; internally the methods will first check if the session is already open.

Accessing Session Data

To access the data stored in session, you can do the following:

$session = Yii::$app->session;

// get a session variable. The following usages are equivalent:
$language = $session->get('language');
$language = $session['language'];
$language = isset($_SESSION['language']) ? $_SESSION['language'] : null;

// set a session variable. The following usages are equivalent:
$session->set('language', 'en-US');
$session['language'] = 'en-US';
$_SESSION['language'] = 'en-US';

// remove a session variable. The following usages are equivalent:
$session->remove('language');
unset($session['language']);
unset($_SESSION['language']);

// check if a session variable exists. The following usages are equivalent:
if ($session->has('language')) ...
if (isset($session['language'])) ...
if (isset($_SESSION['language'])) ...

// traverse all session variables. The following usages are equivalent:
foreach ($session as $name => $value) ...
foreach ($_SESSION as $name => $value) ...

Info: When you access session data through the session component, a session will be automatically opened if it has not been done so before. This is different from accessing session data through $_SESSION, which requires an explicit call of session_start().

When working with session data that are arrays, the session component has a limitation which prevents you from directly modifying an array element. For example,

$session = Yii::$app->session;

// the following code will NOT work
$session['captcha']['number'] = 5;
$session['captcha']['lifetime'] = 3600;

// the following code works:
$session['captcha'] = [
    'number' => 5,
    'lifetime' => 3600,
];

// the following code also works:
echo $session['captcha']['lifetime'];

You can use one of the following workarounds to solve this problem:

$session = Yii::$app->session;

// directly use $_SESSION (make sure Yii::$app->session->open() has been called)
$_SESSION['captcha']['number'] = 5;
$_SESSION['captcha']['lifetime'] = 3600;

// get the whole array first, modify it and then save it back
$captcha = $session['captcha'];
$captcha['number'] = 5;
$captcha['lifetime'] = 3600;
$session['captcha'] = $captcha;

// use ArrayObject instead of array
$session['captcha'] = new \ArrayObject;
...
$session['captcha']['number'] = 5;
$session['captcha']['lifetime'] = 3600;

// store array data by keys with a common prefix
$session['captcha.number'] = 5;
$session['captcha.lifetime'] = 3600;

For better performance and code readability, we recommend the last workaround. That is, instead of storing an array as a single session variable, you store each array element as a session variable which shares the same key prefix with other array elements.

Custom Session Storage

The default [[yii\web\Session]] class stores session data as files on the server. Yii also provides the following session classes implementing different session storage:

  • [[yii\web\DbSession]]: stores session data in a database table.
  • [[yii\web\CacheSession]]: stores session data in a cache with the help of a configured cache component.
  • [[yii\redis\Session]]: stores session data using redis as the storage medium.
  • [[yii\mongodb\Session]]: stores session data in a MongoDB.

All these session classes support the same set of API methods. As a result, you can switch to a different session storage class without the need to modify your application code that uses sessions.

Note: If you want to access session data via $_SESSION while using custom session storage, you must make sure that the session has already been started by [[yii\web\Session::open()]]. This is because custom session storage handlers are registered within this method.

To learn how to configure and use these component classes, please refer to their API documentation. Below is an example showing how to configure [[yii\web\DbSession]] in the application configuration to use a database table for session storage:

return [
    'components' => [
        'session' => [
            'class' => 'yii\web\DbSession',
            // 'db' => 'mydb',  // the application component ID of the DB connection. Defaults to 'db'.
            // 'sessionTable' => 'my_session', // session table name. Defaults to 'session'.
        ],
    ],
];

You also need to create the following database table to store session data:

CREATE TABLE session
(
    id CHAR(40) NOT NULL PRIMARY KEY,
    expire INTEGER,
    data BLOB
)

where 'BLOB' refers to the BLOB-type of your preferred DBMS. Below are the BLOB types that can be used for some popular DBMS:

  • MySQL: LONGBLOB
  • PostgreSQL: BYTEA
  • MSSQL: BLOB

Note: According to the php.ini setting of session.hash_function, you may need to adjust the length of the id column. For example, if session.hash_function=sha256, you should use a length 64 instead of 40.

Flash Data

Flash data is a special kind of session data which, once set in one request, will only be available during the next request and will be automatically deleted afterwards. Flash data is most commonly used to implement messages that should only be displayed to end users once, such as a confirmation message displayed after a user successfully submits a form.

You can set and access flash data through the session application component. For example,

$session = Yii::$app->session;

// Request #1
// set a flash message named as "postDeleted"
$session->setFlash('postDeleted', 'You have successfully deleted your post.');

// Request #2
// display the flash message named "postDeleted"
echo $session->getFlash('postDeleted');

// Request #3
// $result will be false since the flash message was automatically deleted
$result = $session->hasFlash('postDeleted');

Like regular session data, you can store arbitrary data as flash data.

When you call [[yii\web\Session::setFlash()]], it will overwrite any existing flash data that has the same name. To append new flash data to an existing message of the same name, you may call [[yii\web\Session::addFlash()]] instead. For example:

$session = Yii::$app->session;

// Request #1
// add a few flash messages under the name of "alerts"
$session->addFlash('alerts', 'You have successfully deleted your post.');
$session->addFlash('alerts', 'You have successfully added a new friend.');
$session->addFlash('alerts', 'You are promoted.');

// Request #2
// $alerts is an array of the flash messages under the name of "alerts"
$alerts = $session->getFlash('alerts');

Note: Try not to use [[yii\web\Session::setFlash()]] together with [[yii\web\Session::addFlash()]] for flash data of the same name. This is because the latter method will automatically turn the flash data into an array so that it can append new flash data of the same name. As a result, when you call [[yii\web\Session::getFlash()]], you may find sometimes you are getting an array while sometimes you are getting a string, depending on the order of the invocation of these two methods.

Cookies

Yii represents each cookie as an object of [[yii\web\Cookie]]. Both [[yii\web\Request]] and [[yii\web\Response]] maintain a collection of cookies via the property named cookies. The cookie collection in the former represents the cookies submitted in a request, while the cookie collection in the latter represents the cookies that are to be sent to the user.

Reading Cookies

You can get the cookies in the current request using the following code:

// get the cookie collection (yii\web\CookieCollection) from the "request" component
$cookies = Yii::$app->request->cookies;

// get the "language" cookie value. If the cookie does not exist, return "en" as the default value.
$language = $cookies->getValue('language', 'en');

// an alternative way of getting the "language" cookie value
if (($cookie = $cookies->get('language')) !== null) {
    $language = $cookie->value;
}

// you may also use $cookies like an array
if (isset($cookies['language'])) {
    $language = $cookies['language']->value;
}

// check if there is a "language" cookie
if ($cookies->has('language')) ...
if (isset($cookies['language'])) ...

Sending Cookies

You can send cookies to end users using the following code:

// get the cookie collection (yii\web\CookieCollection) from the "response" component
$cookies = Yii::$app->response->cookies;

// add a new cookie to the response to be sent
$cookies->add(new \yii\web\Cookie([
    'name' => 'language',
    'value' => 'zh-CN',
]));

// remove a cookie
$cookies->remove('language');
// equivalent to the following
unset($cookies['language']);

Besides the [[yii\web\Cookie::name|name]], [[yii\web\Cookie::value|value]] properties shown in the above examples, the [[yii\web\Cookie]] class also defines other properties to fully represent all available cookie information, such as [[yii\web\Cookie::domain|domain]], [[yii\web\Cookie::expire|expire]]. You may configure these properties as needed to prepare a cookie and then add it to the response's cookie collection.

Note: For better security, the default value of [[yii\web\Cookie::httpOnly]] is set to true. This helps mitigate the risk of a client side script accessing the protected cookie (if the browser supports it). You may read the httpOnly wiki article for more details.

Cookie Validation

When you are reading and sending cookies through the request and response components as shown in the last two subsections, you enjoy the added security of cookie validation which protects cookies from being modified on the client side. This is achieved by signing each cookie with a hash string, which allows the application to tell if a cookie has been modified on the client side. If so, the cookie will NOT be accessible through the [[yii\web\Request::cookies|cookie collection]] of the request component.

Note: Cookie validation only protects cookie values from being modified. If a cookie fails the validation, you may still access it through $_COOKIE. This is because third-party libraries may manipulate cookies in their own way, which does not involve cookie validation.

Cookie validation is enabled by default. You can disable it by setting the [[yii\web\Request::enableCookieValidation]] property to be false, although we strongly recommend you do not do so.

Note: Cookies that are directly read/sent via $_COOKIE and setcookie() will NOT be validated.

When using cookie validation, you must specify a [[yii\web\Request::cookieValidationKey]] that will be used to generate the aforementioned hash strings. You can do so by configuring the request component in the application configuration:

return [
    'components' => [
        'request' => [
            'cookieValidationKey' => 'fill in a secret key here',
        ],
    ],
];

Info: [[yii\web\Request::cookieValidationKey|cookieValidationKey]] is critical to your application's security. It should only be known to people you trust. Do not store it in the version control system.