Add a note about how the files session backend locks session files.

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@331493 c90b9560-bf6c-de11-be94-00142212c4b1

reference/session/examples.xml

@@ -88,6 +88,24 @@ unset($_SESSION['count']);
     register_globals will overwrite variables in the global scope whose names are shared with session variables. Please see <link linkend="security.globals">Using Register Globals</link> for details.
    </para>
   </warning>
+  <note>
+   <para>
+    File based sessions (the default in PHP) lock the session file once a
+    session is opened via <function>session_start</function> or implicitly via
+    <link linkend="ini.session.auto-start">session.auto_start</link>. Once
+    locked, no other script can access the same session file until it has been
+    closed by the first script terminating or calling
+    <function>session_write_close</function>.
+   </para>
+   <para>
+    This is most likely to be an issue on Web sites that use AJAX heavily and
+    have multiple concurrent requests. The easiest way to deal with it is to
+    call <function>session_write_close</function> as soon as any required
+    changes to the session have been made, preferably early in the script.
+    Alternatively, a different session backend that does support concurrency
+    could be used.
+   </para>
+  </note>
  </section>
 
  <section xml:id="session.idpassing">