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# XLIX. Session handling functions

Session support in PHP consists of a way to preserve certain data across subsequent accesses. This enables you to build more customized applications and increase the appeal of your web site.

If you are familiar with the session management of PHPLIB, you will notice that some concepts are similar to PHP's session support.

A visitor accessing your web site is assigned an unique id, the so-called session id. This is either stored in a cookie on the user side or is propagated in the URL.

The session support allows you to register arbitrary numbers of variables to be preserved across requests. When a visitor accesses your site, PHP will check automatically (if session.auto_start is set to 1) or on your request (explicitly through session_start() or implicitly through session_register()) whether a specific session id has been sent with the request. If this is the case, the prior saved environment is recreated.

All registered variables are serialized after the request finishes. Registered variables which are undefined are marked as being not defined. On subsequent accesses, these are not defined by the session module unless the user defines them later.

There are two methods to propagate a session id:

- Cookies

- URL parameter

The session module supports both methods. Cookies are optimal, but since they are not reliable (clients are not bound to accept them), we cannot rely on them. The second method embeds the session id directly into URLs.

PHP is capable of doing this transparently when compiled with `--enable-trans-sid`. If you enable this option, relative URIs will be changed to contain the session id automatically. Alternatively, you can use the constant `SID` which is defined, if the client did not send the appropiate cookie. `SID` is either of the form `session_name=session_id` or is an empty string.

The following example demonstrates how to register a variable, and how to link correctly to another page using SID.

Example 1. Counting the number of hits of a single user
    <?php

    session_register("count");

    $count++;

    ?>

    Hello visitor, you have seen this page <? echo $count; ?> times.<p>

    <?
    # the <?=SID?> is necessary to preserve the session id
    # in the case that the user has disabled cookies
    ?>

     To continue, <A HREF="nextpage.php?<?=SID?>">click here</A>

To implement database storage you need PHP code and a user level function session_set_save_handler(). You would have to extend the following functions to cover MySQL or another database.

Example 2. Usage of session_set_save_handler()
    <?php

    function open ($save_path, $session_name) {
        echo "open ($save_path, $session_name)\n";
        return true;
    }

    function close () {
        echo "close\n";
        return true;
    }

    function read ($key) {
        echo "write ($key, $val)\n";
        return "foo|i:1;";
    }

    function write ($key, $val) {
        echo "write ($key, $val)\n";
        return true;
    }

    function destroy ($key)
        return true;
    }

    function gc ($maxlifetime) {
        return true;
    }

    session_set_save_handler ("open", "close", "read", "write", "destroy", "gc");

    session_start ();

    $foo++;

    ?>

Will produce this results:

    $ ./php save_handler.php
    Content-Type: text/html
    Set-cookie: PHPSESSID=f08b925af0ecb52bdd2de97d95cdbe6b

    open (/tmp, PHPSESSID)
    read (f08b925af0ecb52bdd2de97d95cdbe6b)
    write (f08b925af0ecb52bdd2de97d95cdbe6b, foo|i:2;)
    close

The `<?=SID?>` is not necessary, if `--enable-trans-sid` was used to compile PHP.

The session management system supports a number of configuration options which you can place in your php.ini file. We will give a short overview.

- `session.save_handler` defines the name of the handler which is used for storing and retrieving data associated with a session. Defaults to `files`.

- `session.save_path` defines the argument which is passed to the save handler. If you choose the default files handler, this is the path where the files are created. Defaults to `/tmp`.

- `session.name` specifies the name of the session which is used as cookie name. It should only contain alphanumeric characters. Defaults to `PHPSESSID`.

- `session.auto_start` specifies whether the session module start a session automatically on request startup. Defaults to `0` (disabled).

- `session.lifetime` specifies the lifetime of the cookie in seconds which is sent to the browser. The value 0 means "until the browser is closed." Defaults to `0`.

- `session.serialize_handler` defines the name of the handler which is used to serialize/deserialize data. Currently, a PHP internal format (name `php`) and WDDX is supported (name `wddx`). WDDX is only available, if PHP is compiled with WDDX support. Defaults to `php`.

- `session.gc_probability` specifies the probability that the gc (garbage collection) routine is started on each request in percent. Defaults to `1`.

- `session.gc_maxlifetime` specifies the number of seconds after which data will be seen as 'garbage' and cleaned up.

- `session.extern_referer_check` determines whether session ids referred to by external sites will be eliminated. If session ids are propagated using the URL method, users not knowing about the impact might publish session ids. This can lead to security problems which this check tries to defeat. Defaults to `0`.

- `session.entropy_file` gives a path to an external resource (file) which will be used as an additional entropy source in the session id creation process. Examples are `/dev/random` or `/dev/urandom` which are available on many Unix systems.

- `session.entropy_length` specifies the number of bytes which will be read from the file specified above. Defaults to `0` (disabled).

- `session.use_cookies` specifies whether the module will use cookies to store the session id on the client side. Defaults to `1` (enabled).

> Note: Session handling was added in PHP 4.0.

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