<!-- All of the content stays in the index -->
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Home Page</title>
</head>
<body>
    <nav>
        <div>
            <a href="index.php?page=welcome">
                Welcome</a>
        </div>
        <div>
            <a href="index.php?page=contact">
                Contact</a>
        </div>
    </nav>
    
    <div id="main">
        <?php
                
if(isset($_GET['page']))
                {
                    
// get content of the paramter
                    
$page $_GET['page'];
                    
// sanitize
                    
$page str_replace('..','',$page);
                    
$page str_replace('/','',$page);
                    
$page str_replace('\\','',$page);
                    
// compose filename
                    
$filename='pages/'.$page.'.php';


                    
// check if the file exists
                    
if(file_exists($filename))
                        
// include the file
                        
include($filename); 
                }
            
?>
    </div>

</body>
</html>