PHP sessions are unsetting when i click href php code

In index.php i am including 2 files
1. header.php
2. mainpage.php

in header .php I wrote code on visit it shows Welcome User
and also header.php have two links
1.login(drop down windows using bootstrap) ..if he press login...login form loads.. and
user fill form and submit..if valid details... user name will print...and
$_SESSION['loggedin'] is setting
2.after using login..mainpage.php will find loggedin session is setted..so some link i am giving
1.<a href="index.php?a='info'" rel="nofollow" target="_blank">information</a>
2.<a href="index.php?a=" rel="nofollow" target="_blank"edit"">Edit</a>

when i lick any one of link.. page is reloading and all session are unsetting..

How can i Over come this problem