I am getting this undefined index error line 23

Im new to PHP and I have been following a tutorial on youtube. I copied the same code showed on youtube and on the video their seemed to be no errors, and when I tried checking the code on virtual studio the PHP debugger showed no mistakes. But when I try checking it on the localhost, this notification shows up. "Notice: Undefined index: heartrate in C:\xampp\htdocs\testcode\trial2.php on line 23". Pls tell me what I’m doing wrong.

This is the code

<?php
$dbuser = 'root';  
$dbpass = '78952'; 
$dbhost = 'localhost';
class thesis_final{
 public $link='';
 function __construct($heartrate){
  $this->connect();
  $this->storeInDB($heartrate);
 }
 
 function connect(){
  $this->link = mysqli_connect('$dbhost','$dbuser','$dbpass') or die('Cannot connect to the DB');
  mysqli_select_db($this->link,'<example>') or die('Cannot select the DB');
 }
 
 function storeInDB($heartrate){
  $query = "insert into <thesis_final> set heartrate='".$heartrate."'";
  $result = mysqli_query($this->link,$query) or die('Errant query:  '.$query);
 }
 
}
if($_GET['heartrate'] != ''){
 $thesis_final=new thesis_final($_GET['heartrate']);
}
?>

>Solution :

Instead of checking if $_GET[‘heartrate’] is ”, use isset.

Example:

if (isset($_GET['heartrate'])) {
    $thesis_final = new thesis_final($_GET['heartrate']);
}