The bit of code you should will only change the title depending on what task is called. By the way...to get the different title to actually display you should use this bit of code:
PHP Code:
echo "<title>$title</title>"; 
within the head tags.
Another thing is...it would be much easier to use a switch statement instead of an if statement in this case. Use this code:
switch($task) {
case "products": $title = "Products"; break;
case "services": $title = "Services"; break;
case "showcase": $title = "Showcase"; break;
case "screenshots": $title = "Screenshots"; break;
case "contact": $title = "Contact Us"; break;
case "about": $title = "About Us"; break;
default:
$title = "Welcome";
}

This script isnt very helpful as it doesnt even print out a page. However you can easily extend it. The easy way to do this task would be to add the line
PHP Code:
<?php include($page); ?>
right after your body tag on the page. Then you would just create all the other pages as their own documents...like about.php, which would have the about page contents. Then, to make the correct page show up you just go to, for example, index.php?page=about.php.