isset kullanarak denersen belki olur.

if(isset($_GET['site'])){

switch($_GET['site'])
{
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'register':
        include_once('sayfalar/register.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'changepass':
        include_once('sayfalar/changepass.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'ep':
        include_once('sayfalar/ep.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'socialid':
        include_once('sayfalar/socialid.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'ranking':
        include_once('sayfalar/ranking.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'lostpw':
        include_once('sayfalar/lostpw.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'sura':
        include_once('sayfalar/sura.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'ninja':
        include_once('sayfalar/ninja.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'savasci':
        include_once('sayfalar/savasci.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'saman':
        include_once('sayfalar/saman.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'hepsi':
        include_once('sayfalar/hepsi.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'rules':
        include_once('sayfalar/rules.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'lostpw_ok':
        include_once('sayfalar/lostpw_ok.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'reg_ok':
        include_once('sayfalar/reg_ok.php');
    break;    
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'error':
        include_once('sayfalar/error.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'userzone':
        include_once('sayfalar/userzone.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'logout':
        include_once('sayfalar/logout.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'download':
        include_once('sayfalar/download.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'team':
        include_once('sayfalar/team.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'market':
        include_once('sayfalar/market.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    case'events':
        include_once('sayfalar/events.php');
    break;
// ----------------------------------------------------------------------------------------------------------------------------------------------//
    default:
        include_once('sayfalar/anasayfa.php');
// ----------------------------------------------------------------------------------------------------------------------------------------------//
} 

}