[php adding and subtracting]

Hi ! please help me . im having a trouble with this im actually new to php. 

it says Notice: Undefined index: prev in C:\xampp\htdocs\new\viewbill.php on line 21

Notice: Undefined index: pres in C:\xampp\htdocs\new\viewbill.php on line 22

Notice: Undefined index: price in C:\xampp\htdocs\new\viewbill.php on line 23

Notice: Undefined index: id in C:\xampp\htdocs\new\viewbill.php on line 28

 

<?php

include 'connection.php';

$id = $_GET['id'];

$result = mysql_query("SELECT * FROM meter WHERE reading_id  = '$id'");

 

 

echo "<table border='1' bgcolor='#fff'>

<tr>

<th>Id</th>

<th>Previous Reading</th>

<th>Present Reading</th>

<th>Consuption</th>

<th>Date</th>

<th>Bill Amount</th>

<th>Action</th>

</tr>";

 

while($row = mysql_fetch_array($result)){

 

 $prev=$row['prev'];

 $pres=$row['pres'];

 $price=$row['price'];

 $totalcons=$pres - $prev;

 $bill=$totalcons * $price;

 

  echo "<tr>";

  echo "<td>" . $row['id'] . "</td>";

  echo "<td>" . $prev . "</td>";

  echo "<td>" . $pres . "</td>";

  echo "<td>". $totalcons."</td>";

  echo "<td>" . $price . "</td>";

  echo "<td>" . $bill . "</td>";

  echo "</tr>";

  }

  

echo "</table>";

 

?>