Do we need the If statements if so why?

I'm not an expert but, my reckoning would be so that there's less calls to the database. First it's checking if there's a value to get, if no value is set then don't run a query on the database. Imagine if you had dozens of products and you ran queries for each and every value be it present or an empty value, it's unnecessary work on the database and server. Thus, slowing things down.

What does Randy say in the video?

The query string could have been removed so you need to be sure the variable has been set before doing anything with it

Without the first two If statement and invalid id, I still got redirected to shits.php when i entered.

I still wonder what steps they take in the background.

Thanks for your help

With PHP it's a good practice to use if (isset()). You might not notice a difference depending on how your PHP interpreter is set up, but you can encounter notice messages on your web page complaining about missing indexes and undefined values.

In this case here - if(isset($_GET['id'])) might seem redundant because you think that 'id' may always exist as a value in the $_GET variable. You might even be right in this particular case. However, PHP pages are sometimes called multiple times using GET or POST, or GET with a different set of parameters. That means that there are going to be cases where the code you have there is run and the PHP interpreter will try to assess the value of $_GET['id'] when it doesn't necessarily exist.

I did a course recently where this came up and Notice messages are produced complaining about missing indexes. The code still actually runs OK, there's just extra junk on the screen. The instructor advised to change the settings of the interpreter so that those notices didn't appear. I don't particularly agree with this advice and think that code which generates warning messages from the interpreter is probably not good code and should be improved, but I can be fairly pedantic that way.

So - in summary. Leaving out some of these conditionals won't necessarily make a difference to the operation of the site, but it's good practice not to make too many assumptions based on input coming into a function/script. I wouldn't personally write the script the way it was presented. I'd probably write something like this:

<?php

    $product_id = isset($_GET["id"]) ? $_GET["id"] : false;
    if ($product_id && isset($products[$product_id])) {
        $product = $products[$product_id];
        header("Location: shirts.php");
        exit();
    } else {
        /* something wrong with input/data */
    }

>?

If I've tested that 'id' was an input parameter and I've established that there is a matching entry in the $products object, I shouldn't need to test isset($product).

Is any of this helping?