i dont know but i tried this and it seem working:
$postvar = array ('1'=>$POST['fullname'],
'2'=>$POST['firstname'],
'3'=>'lastname',
'4'=>'password',
'5'=>'passconfirm',
'6'=>'email',
'7'=>'emailconfirm',
'8'=>'sex',
'9'=>'country');
}
if(isset($postvar['1']) && empty($postvar['1']) ){
echo "<p class='error'>Your name is missing plz fill it again!</p>";
}