Variable errors in session for admin pages - php

Question

I am creating a login for a website. I can get the code below working: It lets me log in! Yet I can't get a start session to work: People can still get to my pages via URL.
Log in PHP:
<?php
//calling connection to database
include "connection.php";
//if user posts for called login
if(isset($_POST['login'])){
//declaring variables for user input and using escape string to protect php scripts
$user = mysqli_real_escape_string($dbconn,$_POST['user']);
$pass = mysqli_real_escape_string($dbconn,$_POST['pass']);
//select from users table where user input matches un and pw
$sel_user = "SELECT * from users where un='$user' AND pw='$pass'";
//put content held in sel_user into variable run_user
$run_user = mysqli_query($dbconn, $sel_user);
//use run_user counting rows and save in check_user
$check_user = mysqli_num_rows($run_user);
//if content row numbers greater than 0
if($check_user>0){
//session where un is equal to user input stored in $user
$_SESSION['username']=$user;
//display admin main page
header('Location: ../adminmain.php');
}
else {
//display log in error page
header('Location: ../loginerror.php');
}
}
//close database connection
mysqli_close($dbconn);
?>
Start session code which says undefined variables:
<?php
include"includes/loginrequiredb.php";
if($_SESSION['username'] !=$user){
session_destroy();
header("Location: view.php");
die();
}else
{
echo "welcome to the site you have logged in" . $_SESSION['username'];
}
?>

Answer 1

Without starting the session you can not get the values from $_SESSION.
You just need to start session in your both files as:
session_start();
Note that you need to start_session() in both files only in just welcome file.
Side note:
I suggest to also use isset() for checking either value set or not.

Answer 2

Start the session with session_start and Add a session verification file in adminmain.php page.
<?php
//calling connection to database
include "connection.php";
#session_start();
//session
//if user posts for called login
if(isset($_POST['login'])){
//declaring variables for user input and using escape string to protect php scripts
$user = mysqli_real_escape_string($dbconn,$_POST['user']);
$pass = mysqli_real_escape_string($dbconn,$_POST['pass']);
//select from users table where user input matches un and pw
$sel_user = "SELECT * from users where un='$user' AND pw='$pass'";
//put content held in sel_user into variable run_user
$run_user = mysqli_query($dbconn, $sel_user);
//use run_user counting rows and save in check_user
$check_user = mysqli_num_rows($run_user);
//if content row numbers greater than 0
if($check_user>0){
//session where un is equal to user input stored in $user
$_SESSION['username']=$user;
//display admin main page
header('Location: ../adminmain.php');
}
else {
//display log in error page
header('Location: ../loginerror.php');
}
}
//close database connection
mysqli_close($dbconn);
?>
##### file verify.php #####
<?php #session_start();
if (#$_SESSION['username']!=$user) {
header ("location: index.php");
exit;
}
?>