How are you using $pageTitle and $section without including the catalog.php where you are setting the variables?

It's being defined in a different file before the header is included. For example, on suggest.php (code below - couldn't paste a screenshot) the $pageTitle variable is defined just before header.php is included:

// From suggest.php
<?php
$pageTitle = "Suggest a Media Item"; ?>

include("inc/header.php");

<div class="section page">
    <h1>Suggest a Media Item</h1>
</div>

<?php include("inc/footer.php"); ?>

Often a lot of programmatic steps will be taken before including the header (setting the page title, setting meta, instantiating objects that will be used throughout the page, etc).

I used to struggle with the idea that header.php should be the first thing and was writing some pretty awful conditional statements to make that work 5 years ago. Thinking of the header as a component rather than following the visual cue from the website that it's the "top of the page" helped break some bad habits for me.

To add to what Mike said above you want to include a reference to the variable in header.php so instead of the hard coded text, you'd echo the variable like this

<h1> <?php> echo $pageTitle <?php? </h1>

That will then pick up the variable defined at the top of on an individual php template e.g. home.php