Custom Login Page — wp_signon Headers Already Sent?

You have to hook earlier in the request to log someone in, you can’t do it from a page template because, as the error says, headers have been sent to the browser already, so it’s too late at that point.

<?php   
add_action('template_redirect', 'my_check_login');

function my_check_login(){
    if( is_page('my-login-page') && isset($_POST['email']) ){
        // check input and
        // do my login stuff here   
    }
}