A simple include script.

gistfile1.php

<?php

function includePage($GetParameter)
{

    if (empty($GetParameter))
    {
        require('inc/defaultpage.php'); // Default page.
    }
    else
    {
        if (file_exists('inc/' . $GetParameter . '.php') && (ctype_alpha($GetParameter))) // Checks if the file exist and that the get parameter only contains characters.
        {
            require('inc/' . $GetParameter . '.php'); // The file exist. Include and display it.
        }
        else 
        {
            require('inc/error.php'); // No page with that name. Include an error page.
        }
    }
}

?>