view_forum.php. Can't fix the get id - php

Question

There's an error at line $id=$_GET['id']; said that Notice: Undefined index: id in D:\XAMPP\htdocs\view_topic.php on line 101. I tried to change " $_GET " to " $_POST " but the error is still the same. Any help ?
I am trying to retrieve the id from the database and listed all the forum topic posted by users. Others php file can run smoothly. I got problem retrieving id of the post.
<?php
$host="localhost";
$username="root";
$password="";
$db_name="db";
$tbl_name="forum_question";
// Connect to server and select databse.
mysql_connect("$host", "$username", "$password")or die("cannot connect");
mysql_select_db("$db_name")or die("cannot select DB");
// get value of id that sent from address bar
$id=$_GET['id'];
$sql="SELECT * FROM $tbl_name WHERE id='$id'";
$result=mysql_query($sql);
$rows=mysql_fetch_array($result);
?>

Answer 1

Always make use of the isset construct when assigning data to variables from outside world
if(isset($_GET['id']))
{
$id=$_GET['id'];
$sql="SELECT * FROM $tbl_name WHERE id='$id'";
$result=mysql_query($sql);
$rows=mysql_fetch_array($result);
}
else
{
echo "ID was not set. Let me go and check the form again !";
}
?>

Answer 2

This variable has to be set in your URL. You have to check if it's present:
<?php
$host = "localhost";
$username = "root";
$password = "";
$db_name = "db";
$tbl_name = "forum_question";
// Connect to server and select databse.
$db = new mysqli($host, $username, $password, $db_name);
// get value of id that sent from address bar
$id = isset($_GET['id']) ? (int) $_GET['id'] : 0;
if($id <> 0) {
// TODO
// 404 Not Found
} else {
$sql = "SELECT * FROM $tbl_name WHERE id='$id'";
$row = $db->query($sql)->fetch_assoc();
// TODO
// Do Something with Data
}
Your URL must then be http://example.com/path/to/script.php?id=42
I added (int), so no sql injections are possible.
I replaced mysql_* by MySQLi, see comment to your question.
I removed quotes from variables in your query, you don't need them.