# The SessionHandler class

&reftitle.intro;

`SessionHandler` is a special class that can be used to expose the current internal PHP session save handler by inheritance. There are seven methods which wrap the seven internal session save handler callbacks (`open`, `close`, `read`, `write`, `destroy`, `gc` and `create_sid`). By default, this class will wrap whatever internal save handler is set as defined by the session.save_handler configuration directive which is usually `files` by default. Other internal session save handlers are provided by PHP extensions such as SQLite (as `sqlite`), Memcache (as `memcache`), and Memcached (as `memcached`).

When a plain instance of `SessionHandler` is set as the save handler using `session_set_save_handler` it will wrap the current save handlers. A class extending from `SessionHandler` allows you to override the methods or intercept or filter them by calls the parent class methods which ultimately wrap the internal PHP session handlers.

This allows you, for example, to intercept the `read` and `write` methods to encrypt/decrypt the session data and then pass the result to and from the parent class. Alternatively one might chose to entirely override a method like the garbage collection callback `gc`.

Because the `SessionHandler` wraps the current internal save handler methods, the above example of encryption can be applied to any internal save handler without having to know the internals of the handlers.

To use this class, first set the save handler you wish to expose using session.save_handler and then pass an instance of `SessionHandler` or one extending it to `session_set_save_handler`.

Please note that the callback methods of this class are designed to be called internally by PHP and are not meant to be called from user-space code. The return values are equally processed internally by PHP. For more information on the session workflow, please refer to `session_set_save_handler`.

&reftitle.classsynopsis;

         SessionHandler

         implements
         SessionHandlerInterface

         SessionIdInterface

        &Methods;

&reftitle.notes;

This class is designed to expose the current internal PHP session save handler, if you want to write your own custom save handlers, please implement the `SessionHandlerInterface` interface instead of extending from `SessionHandler`.

&reftitle.examples;

### Using `SessionHandler` to add encryption to internal PHP save handlers.

    <?php

     /**
      * decrypt AES 256
      *
      * @param data $edata
      * @param string $password
      * @return decrypted data
      */
    function decrypt($edata, $password) {
        $data = base64_decode($edata);
        $salt = substr($data, 0, 16);
        $ct = substr($data, 16);

        $rounds = 3; // depends on key length
        $data00 = $password.$salt;
        $hash = array();
        $hash[0] = hash('sha256', $data00, true);
        $result = $hash[0];
        for ($i = 1; $i < $rounds; $i++) {
            $hash[$i] = hash('sha256', $hash[$i - 1].$data00, true);
            $result .= $hash[$i];
        }
        $key = substr($result, 0, 32);
        $iv  = substr($result, 32,16);

        return openssl_decrypt($ct, 'AES-256-CBC', $key, true, $iv);
      }

    /**
     * crypt AES 256
     *
     * @param data $data
     * @param string $password
     * @return base64 encrypted data
     */
    function encrypt($data, $password) {
        // Generate a cryptographically secure random salt using random_bytes()
        $salt = random_bytes(16);

        $salted = '';
        $dx = '';
        // Salt the key(32) and iv(16) = 48
        while (strlen($salted) < 48) {
          $dx = hash('sha256', $dx.$password.$salt, true);
          $salted .= $dx;
        }

        $key = substr($salted, 0, 32);
        $iv  = substr($salted, 32,16);

        $encrypted_data = openssl_encrypt($data, 'AES-256-CBC', $key, true, $iv);
        return base64_encode($salt . $encrypted_data);
    }

    class EncryptedSessionHandler extends SessionHandler
    {
        private $key;

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

        public function read($id)
        {
            $data = parent::read($id);

            if (!$data) {
                return "";
            } else {
                return decrypt($data, $this->key);
            }
        }

        public function write($id, $data)
        {
            $data = encrypt($data, $this->key);

            return parent::write($id, $data);
        }
    }

    // we'll intercept the native 'files' handler, but will equally work
    // with other internal native handlers like 'sqlite', 'memcache' or 'memcached'
    // which are provided by PHP extensions.
    ini_set('session.save_handler', 'files');

    $key = 'secret_string';
    $handler = new EncryptedSessionHandler($key);
    session_set_save_handler($handler, true);
    session_start();

    // proceed to set and retrieve values by key from $_SESSION

Since this class' methods are designed to be called internally by PHP as part of the normal session workflow, child class calls to parent methods (i.e. the actual internal native handlers) will return &false; unless the session has actually been started (either automatically, or by explicit `session_start`). This is important to consider when writing unit tests where the class methods might be invoked manually.

&reference.session.entities.sessionhandler;