php-doc-en/reference/session/functions/session-id.xml

<?xml version="1.0" encoding="utf-8"?>
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<refentry xml:id="function.session-id" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>session_id</refname>
  <refpurpose>Get and/or set the current session id</refpurpose>
 </refnamediv>
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type class="union"><type>string</type><type>false</type></type><methodname>session_id</methodname>
   <methodparam choice="opt"><type class="union"><type>string</type><type>null</type></type><parameter>id</parameter><initializer>&null;</initializer></methodparam>
  </methodsynopsis>
  <para>
   <function>session_id</function> is used to get or set the session id for
   the current session.
  </para>
  <para>
   The constant <constant>SID</constant> can also be used to
   retrieve the current name and session id as a string suitable for
   adding to URLs. See also <link linkend="ref.session">Session
   handling</link>.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>id</parameter></term>
      <listitem>
       <para>
        If <parameter>id</parameter> is specified and not &null;, it will replace the current
        session id. <function>session_id</function> needs to be called before
        <function>session_start</function> for that purpose. Depending on the
        session handler, not all characters are allowed within the session id.
        For example, the file session handler only allows characters in the
        range <literal>a-z A-Z 0-9 , (comma) and - (minus)</literal>!
       </para>
       <note>
        <simpara>
         When using session cookies, specifying an <parameter>id</parameter>
         for <function>session_id</function> will always send a new cookie
         when <function>session_start</function> is called, regardless if the
         current session id is identical to the one being set.
        </simpara>
       </note>
      </listitem>
     </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   <function>session_id</function> returns the session id for the current
   session or the empty string (<literal>""</literal>) if there is no current
   session (no current session id exists).
   On failure, &false; is returned.
  </para>
 </refsect1>

 <refsect1 role="changelog">
  &reftitle.changelog;
  <informaltable>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>&Version;</entry>
      <entry>&Description;</entry>
     </row>
    </thead>
    <tbody>
     <row>
      <entry>8.0.0</entry>
      <entry>
       <parameter>id</parameter> is nullable now.
      </entry>
     </row>
    </tbody>
   </tgroup>
  </informaltable>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>session_regenerate_id</function></member>
    <member><function>session_start</function></member>
    <member><function>session_set_save_handler</function></member>
    <member>
     <link linkend="ini.session.save-handler">session.save_handler</link>
    </member>
   </simplelist>
  </para>
 </refsect1>
</refentry>
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