if(!isset($_GET['strana']))
{
    $_GET['strana'] = 0;
}
else
{
    $_GET['strana'] = (int)$_GET['strana'];
}

if($_GET['strana'] < 1)
{
    $_GET['strana'] = 1;
}
else if($_GET['strana'] > $broj_strana )
{
    $_GET['strana'] = $broj_strana ;
}