How to debug Instant Payment Notifcation ( IPN ) page paypal

Question

I am unsure how to debug through my queries within my IPN page. As when I send an IPN simulator it just continuously says successful

Here's the code that I would like to test:

if($payment_status=='Completed'){

    $txn_id_check = mysqli_query($mysqli,"SELECT 'transaction_id' FROM 'payment' WHERE 'transaction_id'='".$txn_id."'");
    if(mysqli_num_rows($txn_id_check)!=1){

        if($receiver_email=='fortunefilly@hotmail.com'){

            if($payment_amount=='9.99' && $payment_currency=='GBP'){

                // add txn_id to db
                $query = "INSERT INTO payment (transaction_id, payment_status, users_id) VALUES(?, ?, ?)";
                $statement = $mysqli->prepare($query);
                $statement->bind_param('sss',$txn_id, $payment_status,$id);
                if($statement->execute()){
                print 'Success! ID of last inserted record is : ' .$statement->insert_id .'<br />'; 
                }else{
                die('Error : ('. $mysqli->errno .') '. $mysqli->error);
                }
                $statement->close();
                // update premium to 1 

                $update_premium = mysqli_query("UPDATE 'users' SET is_member ='1' WHERE 'id' ='".$id."'");
            }
        }
    }
}