index.php

<?php
session_start();
if(isset($_SESSION['myusername'])){
    header("location:main.php");
}
?>


main.php

<?php
session_start();
if(!isset($_SESSION['myusername'])){
    header("location:index.php");
}
print_r($_SESSION); die();

//dane do tego nizej

mysql_connect("$host","$username","$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");
?>