Updated UserDoc.md to add a section for increasing session duration

doc/UserDoc.md

@@ -17,5 +17,10 @@ After that, issue the following commands in the mysql prompt.
 
 The first one uses a database called lrr in MySQL.  The second one deletes a record from `users_table` where the student number is 201131129138.
 
+Increasing session duration
+-------------------
 
-*Last modified on 1 June 2020 by Hui*
+By default, the session duration in PHP is set to 1,440 seconds (24 minutes). However, this is not convenient in most software systems. Therefore, we may need to increase the duration to allow users to have more session time. To increase the session duration, we need to edit the variable *session.gc_maxlifetime* in **php.ini**. We can increase its default value to whatever we want (e.g., 7200).
+On Ubuntu, the file is located at */etc/php/7.2/apache2/php.ini*. On XAMPP, the file is located at */xampp/php/php.ini*.
+
+*Last modified on 20 April 2022 by Umar*