undefined variable php updating mysql data

This is the code for attempting to do a update on mysql data errors stating undefined variable

mysql_connect ("localhost", "root", "");
 mysql_select_db("supplierdetails");
  $con = mysql_connect("localhost", "root", "");
    if (!$con)
     {
     die('Could not connect: ' . mysql_error());
     }
 //Run a query 
     $result = mysql_query ("SELECT * FROM users WHERE id= '$id'");
     while ($row = mysql_fetch_array($result))
    {
 $username=$row['username'];
 $password=$row['password'];
   }
 $query = "UPDATE users SET username = '$username', password = '$password' WHERE id           = '$id'";
 $result = @mysql_query($query);
 //Check whether the query was successful or not
 if($result) {  
     header("message= Users Updated");
 }else {
     die("Query failed");
 }
 ?>
 

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