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 <reference id="ref.session">
  <title>Session Handling Functions</title>
  <titleabbrev>Sessions</titleabbrev>
    
  <partintro>
  
   <section id="session.intro">
    &reftitle.intro;
    <para>
     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.
    </para>
    <para>
     A visitor accessing your web site is assigned a unique id, the
     so-called session id. This is either stored in a cookie on the
     user side or is propagated in the URL.
    </para>
    <para>
     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 <link 
     linkend="ini.session.auto-start">session.auto_start</link>
     is set to 1) or on your request (explicitly through
     <function>session_start</function> or implicitly through
     <function>session_register</function>) whether a specific session
     id has been sent with the request. If this is the case, the prior
     saved environment is recreated.
    </para>
     <caution>
      <para>
       If you do turn on <link linkend="ini.session.auto-start">
       session.auto_start</link> then you cannot put  objects into
       your sessions since the class definition has to be
       loaded before starting the session in order to recreate the
       objects in your session.
      </para>
     </caution>
    <para>
     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.
    </para>
    <warning>
     <para>
      Some types of data can not be serialized thus stored in sessions. It
      includes <type>resource</type> variables or objects with circular
      references (i.e. objects which passes a reference to itself to another
      object).
     </para>
    </warning>
    <note>
     <para>
      Session handling was added in PHP 4.0.
     </para>
    </note>
    <note>
     <para>
      Please note when working with sessions that a record of a session
      is not created until a variable has been registered using the
      <function>session_register</function> function or by adding a new 
      key to the <varname>$_SESSION</varname> superglobal array. This 
      holds true regardless of if a session has been started using the 
      <function>session_start</function> function. 
     </para>
    </note>
   </section>
   
   <section id="session.security">
    <title>Sessions and security</title>
    <para>
     External links: <ulink url="&url.session-fixation;">Session fixation</ulink>
    </para>
    <para>
     The session module cannot guarantee that the information you store
     in a session is only viewed by the user who created the session. You need
     to take additional measures to actively protect the integrity of the
     session, depending on the value associated with it.
    </para>
    <para>
     Assess the importance of the data carried by your sessions and deploy
     additional protections -- this usually comes at a price, reduced
     convenience for the user.  For example, if you want to protect users from
     simple social engineering tactics, you need to enable
     <literal>session.use_only_cookies</literal>.  In that case,
     cookies must be enabled unconditionally on the user side, or
     sessions will not work.
    </para>
    <para>
     There are several ways to leak an existing session id to third parties.
     A leaked session id enables the third party to access all resources which
     are associated with a specific id.  First, URLs carrying session ids.  If
     you link to an external site, the URL including the session id might be
     stored in the external site's referrer logs. Second, a more active
     attacker might listen to your network traffic. If it is not encrypted,
     session ids will flow in plain text over the network. The solution here
     is to implement SSL on your server and make it mandatory for users.
    </para>
   </section>
   <section id="session.requirements">
    &reftitle.required;
    &no.requirement;
    <note>
     <para>
      Optionally you can use shared memory allocation (mm), developed by
      Ralf S. Engelschall, for session storage. You have to download
      <ulink url="&url.mm;">mm</ulink> and install it. This option is not
      available for Windows platforms. Note that the session storage module
      for mm does not guarantee that concurrent accesses to the same session
      are properly locked. It might be more appropriate to use a shared memory
      based filesystem (such as tmpfs on Solaris/Linux, or /dev/md on BSD) to
      store sessions in files, because they are properly locked.
      Session data is stored in memory thus web server restart deletes it.
     </para>
    </note>
   </section>

   &reference.session.configure;

   &reference.session.ini;

   <section id="session.resources">
    &reftitle.resources;
    &no.resource;
   </section>

   &reference.session.constants;

   <section id="session.examples">
    &reftitle.examples;
    <note>
     <para>
      As of PHP 4.1.0, <varname>$_SESSION</varname> is available as a 
      global variable just like <varname>$_POST</varname>,
      <varname>$_GET</varname>, <varname>$_REQUEST</varname> and so on.
      Unlike <varname>$HTTP_SESSION_VARS</varname>,
      <varname>$_SESSION</varname> is always global. Therefore, you do not
      need to use the <link
      linkend="language.variables.scope"><command>global</command></link>
      keyword for <varname>$_SESSION</varname>. Please note that this
      documentation has been changed to use
      <varname>$_SESSION</varname> everywhere. You can substitute
      <varname>$HTTP_SESSION_VARS</varname> for
      <varname>$_SESSION</varname>, if you prefer the former.  Also note
      that you must start your session using <function>session_start</function> 
      before use of <varname>$_SESSION</varname> becomes available.
     </para>
     <para>
      The keys in the <varname>$_SESSION</varname> associative
      array are subject to the
      same limitations as regular variable names in PHP, i.e. they cannot
      start with a number and must start with a letter or underscore.
      For more details see the section on
      <link linkend='language.variables'>variables</link> in this manual.
     </para>
    </note>

    <para>
     If <link
     linkend="ini.register-globals">register_globals</link>
     is disabled, only members of the global associative array
     <varname>$_SESSION</varname> can be registered as session
     variables. The restored session variables will only be available
     in the array <varname>$_SESSION</varname>.
    </para>
    <para>
     Use of <varname>$_SESSION</varname> (or
     <varname>$HTTP_SESSION_VARS</varname> with PHP 4.0.6 or less) is
     recommended for improved security and code readability. With
     <varname>$_SESSION</varname>, there is no need to use the
     <function>session_register</function>,
     <function>session_unregister</function>,
     <function>session_is_registered</function> functions. Session variables
     are accessible like any other variables.
     <example>
      <title>
       Registering a variable with <varname>$_SESSION</varname>.
      </title>
      <programlisting role="php">
<![CDATA[
<?php
session_start();
// Use $HTTP_SESSION_VARS with PHP 4.0.6 or less
if (!isset($_SESSION['count'])) {
    $_SESSION['count'] = 0;
} else {
    $_SESSION['count']++;
}
?>
]]>
      </programlisting>
     </example>
     <example>
      <title>
       Unregistering a variable with <varname>$_SESSION</varname> and
       <link
       linkend="ini.register-globals">register_globals</link> disabled.
      </title>
      <programlisting role="php">
<![CDATA[
<?php
session_start();
// Use $HTTP_SESSION_VARS with PHP 4.0.6 or less
unset($_SESSION['count']);
?>
]]>
      </programlisting>
     </example>
    </para>
    <para>
     <caution>
      <para>
       Do NOT unset the whole <varname>$_SESSION</varname> with
       <literal>unset($_SESSION)</literal> as this will disable the
       registering of session variables through the
       <varname>$_SESSION</varname> superglobal.
      </para>
     </caution>
    </para>
    <warning>
     <para>
      You can't use references in session variables as there is no feasible way
      to restore a reference to another variable.
     </para>
    </warning>
    <para>
     If <link
     linkend="ini.register-globals">register_globals</link>
     is enabled, then each global variable can be registered as session
     variable. Upon a restart of a session, these variables will be restored
     to corresponding global variables. Since PHP must know which global
     variables are registered as session variables, users need to register
     variables with <function>session_register</function> function.
     You can avoid this by simply setting entries in
     <varname>$_SESSION</varname>.
     <caution>
      <para>
       Before PHP 4.3, if you are using <varname>$_SESSION</varname> and you
       have disabled <link linkend="ini.register-globals">register_globals</link>,
       don't use <function>session_register</function>,
       <function>session_is_registered</function> or
       <function>session_unregister</function>.
        Disabling <link
        linkend="ini.register-globals">register_globals</link>
        is recommended for both security and performance reasons.
      </para>
     </caution>
    </para>
    <para>
     If <link
     linkend="ini.register-globals">register_globals</link>
     is enabled, then the global variables and the
     <varname>$_SESSION</varname> entries will automatically reference the
     same values which were registered in the prior session instance.
     However, if the variable is registered by <varname>$_SESSION</varname>
     then the global variable is available since the next request.
    </para>
    <para>
     There is a defect in PHP 4.2.3 and earlier.  If you register a new
     session variable by using <function>session_register</function>, the
     entry in the global scope and the <varname>$_SESSION</varname> entry will
     not reference the same value until the next
     <function>session_start</function>.  I.e. a modification to the newly
     registered global variable will not be reflected by the
     <varname>$_SESSION</varname> entry.  This has been corrected in PHP 4.3.
    </para>
   </section>
    
   <section id="session.idpassing">
    <title>Passing the Session ID</title>
    <para>
     There are two methods to propagate a session id:
     <itemizedlist>
      <listitem>
       <simpara>
        Cookies
       </simpara>
      </listitem>
      <listitem>
       <simpara>
        URL parameter
       </simpara>
      </listitem>
     </itemizedlist>
    </para>
    <para>
     The session module supports both methods. Cookies are optimal, but
     because they are not always available, we also provide an alternative
     way.  The second method embeds the session id directly into URLs.
    </para>
    <para>
     PHP is capable of transforming links transparently. Unless you are using
     PHP 4.2 or later, you need to enable it manually when building PHP.
     Under Unix, pass <link linkend="ini.session.use-trans-sid">
     --enable-trans-sid</link> to configure. If this build
     option and the run-time option
     <literal>session.use_trans_sid</literal> are enabled, relative
     URIs will be changed to contain the session id automatically.
     <note>
      <para>
      The <link linkend="ini.arg-separator.output">arg_separator.output</link>
      &php.ini; directive allows to customize the argument seperator. For full
      XHTML conformance, specify &amp;amp; there.
      </para>
     </note> 
    </para>
    <para>
     Alternatively, you can use the constant <literal>SID</literal> which is
     defined if the session started.  If the client did not send an appropriate session
     cookie, it has the form <literal>session_name=session_id</literal>.
     Otherwise, it expands to an empty string. Thus, you can embed it
     unconditionally into URLs.
    </para>
    <para>
     The following example demonstrates how to register a variable, and
     how to link correctly to another page using SID.
     <example>
      <title>Counting the number of hits of a single user</title>
      <programlisting role="php">
<![CDATA[
<?php
if (!session_is_registered('count')) {
    session_register('count');
    $count = 1;
} else {
    $count++;
}
?>

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

<p>
To continue, <a href="nextpage.php?<?php echo strip_tags(SID); ?>">click
here</a>.
</p>
]]>
      </programlisting>
     </example>
    </para>
    <para>
     The <function>strip_tags</function> is used when printing the SID 
     in order to prevent XSS related attacks.
    </para>
    <para>
     Printing the SID, like shown above, is not necessary if
     <link linkend="ini.session.use-trans-sid">
     --enable-trans-sid</link> was used to compile PHP.
    </para>
    <note>
     <para>
      Non-relative URLs are assumed to point to external sites and
      hence don't append the SID, as it would be a security risk to
      leak the SID to a different server.
     </para>
    </note>
   </section>
   
   <section id="session.customhandler">
    <title>Custom Session Handlers</title>
    <para>
     To implement database storage, or any other storage method, you
     will need to use <function>session_set_save_handler</function> to
     create a set of user-level storage functions.
    </para>
   </section>
  </partintro>

&reference.session.functions;

 </reference>

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