php-doc-en/reference/session/configure.xml

<?xml version="1.0" encoding="utf-8"?>
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<section xml:id="session.installation" xmlns="http://docbook.org/ns/docbook">
 &reftitle.install;
 <para>
  Session support is enabled in PHP by default. If you would
  not like to build your PHP with session support, you should
  specify the <option role="configure">--disable-session</option>
  option to configure. To use shared memory allocation (mm) for session
  storage configure PHP <option role="configure">--with-mm[=DIR] </option>.
 </para>
 &windows.builtin;
 <note>
  <para>
   By default, all data related to a particular session will be stored in
   a file in the directory specified by the session.save_path INI option.
   A file for each session (regardless of if any data is associated with
   that session) will be created. This is due to the fact that a session
   is opened (a file is created) but no data is even written to that file.
   Note that this behavior is a side-effect of the limitations of working
   with the file system and it is possible that a custom session handler
   (such as one which uses a database) does not keep track of sessions
   which store no data. 
  </para>
 </note> 
</section>

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