Something

<?php
    $pages = array(
        'addprograms' => 'addprograms.php',
        'adoptions' => 'adoptions.php',
        'citizens' => 'citizens.php',
        'main' => 'main.php',
        'news' => 'news.php',
        'staff' => 'staff.php',
        'trainingschedule' => 'trainingschedule.php',
        'wanted' => 'wanted.php',
        );
            if (isset($_GET['x']) && isset($pages[$_GET['x']]))
                {
                    include($pages[$_GET['x']]);

                    } else {
                        include($pages['main']);
                }
?>