Session is destroyed on page refresh in PHP - php

Question

I am developing a web application using PHP in which a user initially has to sign in and view his content. I am using PHP sessions to maintain state. I encountered following problems:
Although I started the session on each page and after relevent session variables are set, the session is destroyed each time the page is refreshed or when I browse the same URL on a different tab.
I need the user to be redirected to his content page when the user browsed login page with he has already logged in.
I'm really new to PHP, So I have no idea how to solve these problems. I referred several questions in the stackoverflow, but they all say that sessions are not destroyed on page refresh. I could not understand what's wrong with my page. Any solution with explaination is greatly appreciated.
Login page
<?php
session_start();
class Sessions{
public static function setSessionState($userdata){
unset($userdata['password']);
unset($userdata['timestamp']);
$_SESSION['user']=$userdata;
}
}
if(isset($_POST['username']) && isset($_POST['password'])){
$dbcon = new DBConnection();
$dbcon->connect();
$username= strip_tags(stripslashes(trim($_POST['username'])));
$password = strip_tags(stripcslashes($_POST['password']));
echo "<script>alert($username);</script>";
$result = $dbcon->getUser($username,$password);
if(mysqli_num_rows($result)==1){
$user = $dbcon->getUserData($result); #getUserData function accepts mysqli result as an input and returns a row(array) of user details.
if(isset($user)){
Sessions::setSessionState($user);
header("location:index.php");
}
else{
echo "user variable is not set!!!";
}
}
else if(mysqli_num_rows($result)==0){
echo "Login error! Username or Password incorrect!";
}
else{
die("Unknown Error occured!");
}
}
............
Index page(in which user's private content is visible)
<?php
session_start();
if(isset($_SESSION['user'])){
print_r($_SESSION['user']);
}
else{
echo "session variable not set";
}
?>
Thank you.

Answer 1

I finally found the answer which is actually my bad. I didn't mention the last part of the index.php file as I though that part is irrelevant.In that part I have a part,
<form action="<?php session_destroy(); ?>">
After commenting that session_destroy() method call, I could solve my problem and keep session alive.
Sorry for incomplete code.

Answer 2

try this
class Sessions{
public static function setSessionState($userdata){
if ( !isset($_SESSION['user']) ) {
$_SESSION['user'] = $userdata;
}
}
}