Your page should define its own title. But, if you really want to do it this way, here's a possible solution:
whatever_page_you_load.php:
define("PAGE", "WHATEVERPAGE");
include("header.php");
header.php:
$page_names = array(
"HOME" => "Home Page",
"WHATEVERPAGE" => "Whatever Page's Name";
);
$title = "Default Title";
if(defined("PAGE") && !empty($page_names[PAGE])) {
$title = $page_names[PAGE];
}
echo "<title>" . htmlentities($title) . "</title>";