Looking for a simple approach for handling user $_POST data without AJAX?

I’m looking for a simple way for a WordPress plugin to process $_POST data from a user submitted form. Back end processing will be performed and then I’ll redirect to a new page as appropriate — so during processing there is no UI.

Basically, I want to take a form submit such as…

   <form method="post" action="/back-end-processing.php">
     <input type="text" name="text" />
     <input type="submit" value="submit" />
   </form>';

…and process it like I would with PHP but still be able to use WP’s “nonce” protection.

Some examples I’ve seen suggest that I should use admin-post.php but I was concerned that it should only be used for actual admins and not for general users.

Another approach suggested…

function checkCheckBox()
{
    if($_POST['checkbox'] == 1)
    {
       return true;
    }

    return false;
}

add_action('init', 'checkCheckBox');

…however, this seems like overkill because it will be called for every page while my site visitors will only be posting data to it infrequently.

The best way to process the custom form is the following. If you are using nonce then you don’t really have to check $_POST[‘checkbox’], the code below can be used simply to verify_nonce and then process the form.

function process_my_form() {
    if ( wp_verify_nonce( $_POST['my_nonce_field'], 'my_nonce' ) ) {
        // process your form here
        // you can also redirect and call exit here.
    }
}
add_action( 'init', 'process_my_form' );

But if you still you want to submit your form to a custom php script, then load the wp-blog-header.php.

<?php 
define('WP_USE_THEMES', false);
require('./wp-blog-header.php');
?>

See http://codex.wordpress.org/Integrating_WordPress_with_Your_Website for more information.