Use wp_redirect without filter in plugin file

Question

I created a little private plugin for me, which contains a form, that saves data to a database.

Every user should only have one dataset. If he already has one, he should only be able to edit his current one (similar to profile settings).

Thats no problem, works great. But I have one issue: I want to redirect a user to his edit page, if he views the main plugin page (like site.com/submit/ -> site.com/submit/?id=das12d&edit=1)

So building this url is also no problem. But here is the real issue: I tried using this:

wp-redirect( $url );

And

header('Location: '.$url);

But both causing a Cannot modify header information - headers already sent by... error

How can I solve it?

edit: Some sample code:

function login_data() {
    global $current_user;

    if (is_user_logged_in()) {
        if (isset($_GET['id'])) {
            $id = $_GET['id'];
            show_submission($id);
        } else {
            } elseif(has_submission()) {
                $url = 'http://'.$_SERVER["HTTP_HOST"].$_SERVER["REQUEST_URI"].'?id='.$get_submission_id();
                header('Location: '.$url);
            } else {
                show_form();
            }
        }
    }
}

Answer 1

Not too sure what is really going on there, but one thing for sure your if statement is flawed, you should remove unneeded ELSE, should be like this:

if (is_user_logged_in()) {
        if (isset($_GET['id'])) {
            $id = $_GET['id'];
            show_submission($id);
        } elseif(has_submission())  
             {
                $url = 'http://'.$_SERVER["HTTP_HOST"].$_SERVER["REQUEST_URI"].'?id='.$get_submission_id();
                header('Location: '.$url);
            } else {
                show_form();
            }
        }

OR like this:

if (is_user_logged_in()) {
        if (isset($_GET['id'])) {
            $id = $_GET['id'];
            show_submission($id);
        } else {
             if(has_submission()) {
                $url = 'http://'.$_SERVER["HTTP_HOST"].$_SERVER["REQUEST_URI"].'?id='.$get_submission_id();
                header('Location: '.$url);
            } else {
                show_form();
            }
        }
    }

Maybe this will fix the error