PHP using Includes, how do I create links?

[rascalflattsrule]

I'm building a website in PHP, and I'm new to PHP. I've read that the best way to do it is to have a main file that includes the header.php and footer.php files, then calls a page using if statements and includes. My question is, what do you do for links? Let say I have my main file as index.php, I want to call aboutus.php if the About Us link is clicked, otherwise it will call home.php. What would the About Us link URL be?

[Top Of My Head]

<div id="header">
<?php
include('header.php');
?>
</div>

<div id="header">
<?php
include('body.php');
?>
</div>

<div id="footer">
<?php
include('footer.php');
?>
</div>

-Billy

[Top Of My Head]

<div id="header">
<?php
include('header.php');
?>
</div>

<div id="header">
<?php
include('body.php');
?>
</div>

<div id="footer">
<?php
include('footer.php');
?>
</div>

-Billy

[Top Of My Head]

<div id="header">
<?php
include('header.php');
?>
</div>

<div id="header">
<?php
include('body.php');
?>
</div>

<div id="footer">
<?php
include('footer.php');
?>
</div>

-Billy

[Top Of My Head]

<div id="header">
<?php
include('header.php');
?>
</div>

<div id="header">
<?php
include('body.php');
?>
</div>

<div id="footer">
<?php
include('footer.php');
?>
</div>

-Billy

[Top Of My Head]

<div id="header">
<?php
include('header.php');
?>
</div>

<div id="header">
<?php
include('body.php');
?>
</div>

<div id="footer">
<?php
include('footer.php');
?>
</div>

-Billy

[Top Of My Head]

<div id="header">
<?php
include('header.php');
?>
</div>

<div id="header">
<?php
include('body.php');
?>
</div>

<div id="footer">
<?php
include('footer.php');
?>
</div>

-Billy