Assuming you're using cookies for login, just call it after your setcookie
call -- after all, you must be calling that one before any output too.
Anyway in general you could check for the presence of your form's submit button name at the beginning of the script, do your logic, and then output stuff:
if(isset($_POST['mySubmit'])) {
// the form was submitted
// ...
// perform your logic
// redirect if login was successful
header('Location: /somewhere');
}
// output your stuff here