The way that your system is setup (which is a common method), you are outputting your included page after the beginning of the <body> section, so you can no longer output a page title. This is the same problem that happens with Javascript, CSS and other items that should go in the head.
There are two ways to solve this:
1. Approach this as a two-step process: first set the page title using the information from get, within your index page, then include the content (body) of the page later.
2. The only way to "really" do this, by using a single page, is to create a basic templating system where you output different parts of the page at one time.
Here's a very basic idea for version (2):
In your included pages, set the variables $title and $body to be your page's title and body text, respectively.
Then echo those variables within your template on the index page.
There are of course much more common methods for all of this and those might be more helpful here (but also harder to create).
If you can avoid dealing with templates and use a method like explained by Beverley, that's probably the best way to do this.
