Struggling with Admin check on session variable - php

Question

I have 2 pages. I need on "addproduct.php" to check whether the user is logged in as an admin. I have a login script. Apologies if this is a silly question but im brand new to PHP.
I want a user who reaches this page who is not logged in as an admin ('isadmin' is a row in the user database) to be redirected to the login page, and when someone is logged in as an admin for the page to display.
Login.php;
<?php
session_start();
$un = $_POST["username"];
$pw = $_POST["password"];
$conn = new PDO ("mysql:host=localhost;dbname=assign026;", "assign026",
"ziSietiu");
$results = $conn->query("select * from users where username='$un' and
password='$pw'");
$row = $results->fetch();
if($row == false)
{
echo "Incorrect password!";// There were matching rows
}
else
{
$_SESSION["gatekeeper"] = $un;
$_SESSION["isadmin"] = $row["isadmin"];
header ("Location: index.php");
}
?>
And addproduct.php
<?php
session_start();
?>
<?php
// Test that the authentication session variable exists
if(!isset($_SESSION["isadmin"]) || $row["isadmin"] == 1)
{
header('Location: login.html');
exit();
}
else
{
echo ($_SESSION["isadmin"]);
}
?>
<div>
<h2>Add new product</h2>
<form method="post" action="addproductscript.php">
<p>Insert product here</p>
<input type="text" name="name" placeholder="name">
<input type="text" name="manufacturer" placeholder="manufacturer">
<input type="text" name="description" placeholder="description">
<input type="text" name="price" placeholder="price">
<input type="text" name="stocklevel" placeholder="stocklevel">
<input type="text" name="agelimit" placeholder="agelimit">
<input type="submit" value="Submit">
</form>
</div>

Answer 1

Based on your code if(!isset($_SESSION["isadmin"]) || $row["isadmin"] == 1), the $row["isadmin"] is not defined thus it don't have any value.
What you can do is if(!isset($_SESSION["isadmin"]) || $_SESSION["isadmin"] == 1)