Replace

Code:
$smarty->display('./templates/header.tpl');
$smarty->display('./templates/mypage.tpl');
$smarty->display('./templates/footer.tpl');

With

Code:
$smarty->display('header.tpl');
$smarty->display('mypage.tpl');
$smarty->display('footer.tpl');
Don't have to specify templates folder, that is default smarty tpl folder.