Undefined index PHP error

I've been trying to make a login page with 000webhost, and this error keeps popping up, and i know there are tons of these king of questions everywhere but i still cant fix it

this is the error

Notice: Undefined index: active in /storage/ssd1/326/5052326/public_html/login.php on line 14

Warning: Cannot modify header information - headers already sent by (output started at /storage/ssd1/326/5052326/public_html/login.php:14) in /storage/ssd1/326/5052326/public_html/login.php on line 23

and this is the code

<?php
   include("config.php");
   session_start();
   
      // username and password sent from form 
    if($_SERVER["REQUEST_METHOD"] == "POST") {
          
      $myusername = mysqli_real_escape_string($db,$_POST['username']);
      $mypassword = mysqli_real_escape_string($db,$_POST['password']); 
      
      $sql = "SELECT * FROM admin WHERE username = '$myusername' and password = '$mypassword'";
      $result = mysqli_query($db, $sql);
      $row = mysqli_fetch_array($result,MYSQLI_ASSOC);
      $active = $row['active'];
      
      $count = mysqli_num_rows($result);
      
      // If result matched $myusername and $mypassword, table row must be 1 row
		
      if($count == 1) {
         $_SESSION['login_user'] = $myusername;
         
         header("location: welcome.php");
      }else {
         $error = "Your Login Name or Password is invalid";
      }
    }
?>

What I have tried:

even my teacher couldnt fix it

Tags: PHP