Why does page title have to be defined before the "include" command?

Pretend your include is just that straight code, instead of being an include (because that's what "including" does; anything in that included file is included just exactly as it is in the file, right where you put the include). If it was just the straight code, it might be like this:

<?php $pageTitle = "Contact Mike"; ?>

<?php $echo pageTitle; ?>

If those two code blocks were reversed, you'd be trying to "echo" the value of variable that hadn't been defined yet. First, it must be defined, in order for its value can be called upon later on in the code.

Let's go back to this example, but using the include:

<?php $pageTitle = "Contact Mike"; ?>

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

Make more sense?

It's like giving you water before giving you a bucket. You want to declare your variable first so that the pages loaded afterwards will know what that variable is. (Just thought I'd give a brief answer).

The code is executed in order from top to the bottom. If the variable is declared after the include file, the include file won't know what the variable is because the include file code was run before the variable was declared.