Sideverifisering i php

<?php

    /**
     * [loadPage determines what page should be loaded]
     * @return [string]
     */
    function loadPage()
    {
        global $_GET;

        //Verify that the data about the site actually exists.
        if (!isset($_GET['side']))
        {
            return 'page_default';
        }

        //I hate using get-variables, store in variable
        $userSuppliedPage = $_GET['side'];

        //Verify that the page is allowed, do this by comparing it to an array with allowed pages.
        $allowedPages = array('side1', 'side2', 'side3', 'side4');
        if (!in_array($userSuppliedPage, $allowedPages))
        {
            //User tried to navigate to a webpage that ain't allowed, give 404-error.
            return 'errors/404';
        }

        //Verify that the page actually exists.
        if (!file_exists('pagefiles/' . $userSuppliedPage . '.php'))
        {
            //File doens't exist. Give a 404-error.
            return 'errors/404';
        }

        //File exists and is valid. Security check is OK.
        return $userSuppliedPage;

    }
?>