我想检查一个表的列值在php中不等于零

Question

Database table 我想检查一个表的列值在php中不等于零。但它在列名“未定义的索引”上给出了错误。请指导我的代码有什么问题。我的PHP代码如下所示

<?php 
if($_SERVER['REQUEST_METHOD']=='POST'){
     require_once('dbConnect.php');
     $paydue=$_POST['paydue'];
     $rid=$_POST['rid'];            
     $cardid  = $_POST['cardid'];
     $sq = "SELECT * FROM bill WHERE refid='".$rid."'";
     $query=mysqli_query($con,$sq); 
     $result=mysqli_fetch_row($query); 
     if($result["paydue"] != '0'){
          $sql = "SELECT * FROM account WHERE cardid='".$cardid."'";
          $query1=mysqli_query($con,$sql); 
          $result1=mysqli_fetch_row($query1); 
          $sql1="UPDATE account SET balance = (balance - $paydue) WHERE cardid='$cardid';";
          if(mysqli_query($con,$sql1)){
               echo 'Payed Successfully';
               $sql2="Update bill set paydue = (paydue - $paydue) where refid='$rid';";
               mysqli_query($con,$sql2);
          }else{
               echo 'Could Not Payed Try Again';
          }
      }else{
           echo 'your Bill already Payed';
      } 
      //closing connection 
      mysqli_close($con);
 }
 ?>