Wordpress - wp_redirect() - headers already sent

Found the answer (via)

Instead of using the function I added an action to "wp_loaded", that makes sure that it gets loaded before any headers are sended.

<?php
add_action ('wp_loaded', 'my_custom_redirect');
function my_custom_redirect() {
    if ( isset( $_POST['subscribe'] ) ) {
        $redirect = 'http://example.com/redirect-example-url.html';
        wp_redirect($redirect);
        exit;
    }
}     
?>

You have to use wp_redirect() before get_header() Then it will not show header error.

add_action('template_redirect', function(){
if(isset($_POST['subscriptio'])){// make this condition such that it only matches when a registraiotn form get submitted
/**
 * do your stuff here
 */
wp_redirect();//....
}
});