How to send "Location" header on plugin form submit event?

Question

How to redirect to specified page after form was submitted?

Warning: Cannot modify header information - headers already sent by (output started at wordpress\wp-admin\menu-header.php:94) in wordpress\wp-content\plugins\test\test.php on line 16

<?php
/*
Plugin Name: TEST
Plugin URI: http://localhost/
Description: TEST
Version: 0.0.1
Author: TEST
Author URI: http://localhost/
*/
add_action('admin_menu', 'test');
function test() { 
    add_menu_page('test', 'test', 0, 'test/test.php', 'page');
} 
function page() {
    if(isset($_POST['submit'])) {
        header('Location: /index.php');
        exit;
    }
    echo '<form action="" method="post"><input type="submit" value="Submit form" name="submit" /></form>';
    var_dump($_POST);
}

Answer 1

You're trying to send a redirect header on the admin_menu action, which is fired after some of the page has been sent to the browser. You have to hook an earlier action before output starts, like init.

add_action( 'init', 'wpa_test' );
function wpa_test() {
    if(isset($_POST['submit'])) {
        wp_redirect( home_url() );
        exit;
    }
}