Session is destroyed on page refresh in 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:

1. 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.
2. 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

try this

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

Answer 2

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.