I am not understanding the priority of a task in the Build a basic PHP website. could anyone explain in detail?

Hey there I am currently working on the Build a basic PHP website and have stumbled across something I do not understand. I am currently up to the stage where I am using the catalog.php page to be able to display Movies, Books or Music, depending what link is clicked on the header. I am slightly confused by the code priority it has been implemented in and curious to know why it still works.

In the project video it states putting the code in the following order:

if ($_GET["cat"] == "books"){
  $pageTitle = "Books";
  $section = "books";
} else if ($_GET["cat"] == "movies") {
  $pageTitle = "Movies";
  $section = "movies";
} else if ($_GET["cat"] == "music") {
  $pageTitle = "Music";
  $section = "music";
}

include("inc/header.php"); 

It is this order which is confusing me. It is my understanding that the $_GET["cat"] code is obtaining what was placed in the links of the header.php file. It is checking to see if "cat" equals either books, music or movies and then changing the page to meet the needs of the IF statement.

However my question is this: How is it doing this? Because shouldn't the include("inc/header.php"); go before the above If statement so it knows what link has been clicked upon and then can show the relevant section of either books, music or movies? How comes it still works as surely it should be loaded before this IF statement is executed?