having trouble to understand the concept of product_id = $_GET["id"];

Prakhar,

To expand on what Guan is stating: When you send any information from a form, there are two methods of doing so: post and get. POST sends all the information obtained in the form in the headers of the page while all information sent using GET is sent via the URL. If you remember from looking at the search page, when you did the form, you set the METHOD to get. When you are dealing with the form data on the target page, if you want to access the data if you set the form method to post, you would use $_POST. At the same time, if you use the METHOD get, you would use $_GET as specified in the video.

METHOD: GET

How It's Set

<form action="search.php" method="get">
   <input type="text" id="query" name="q">
</form>

How It's Transferred

http://www.website.com/search.php?q=red

How It's Accessed

<?php
$_GET['q'];
?>

METHOD: POST

How It's Set

<form action="search.php" method="post">
   <input type="text" id="query" name="q">
</form>

How It's Transferred

http://www.website.com/search.php (Remember it's sent via page headers and not from the URL)

How It's Accessed

<?php
$_POST['q'];
?>

You would use POST if you are sending sensitive information or a lot of information from a form. You would use GET if you are sending just a few pieces of information. I hope this helps you better understand the difference.

Cheers!