How to implement post/redirect/get pattern on contact form

Question

I have a shortcode which generates a simple contact form. The form posts back to the same page where its processed, validated and sent via email. All works well, however refreshing the page resubmits the form.

I understand I need a post/redirect/get pattern to prevent this, but can't figure out how to implement this with Wordpress - what should the action be? Can someone provide a simple example?

$errors = [];
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
    //check and validate POST variables
    ...
    // add errors to $errors array
    ...
    // check if form considered spam    
    if(empty($errors) && !$spam) {
        $sent = wp_mail($to, $subject, $message, $headers);
        if($sent) {
            $success = true; 
            $_POST = array();
        }
    } // end if no errors
    echo "<pre>".print_r($_POST, true)."</pre>";
} //end if POST

//loop through errors (if any) and display accordingly
...
// display success message if $success
...

<?php if( !$sent) { //hide form if successfully sent ?>
    <form  id="contact_form" action="" method="post">
        <!-- form inputs -->
    </form>
<?php } //end if not sent ?>

Answer 1

You could use the admin_post URL and actions to process form input and then redirect back to your page.

The form:

<form action="<?php echo admin_url('admin-post.php'); ?>" method="post">
  <input type="hidden" name="action" value="do_something">
  <input type="hidden" name="origin" value="<?php the_ID(); ?>">
  <input type="submit" value="Submit">
</form>

Then the action:

add_action( 'admin_post_do_something', 'wpd_do_something' );
add_action( 'admin_post_nopriv_do_something', 'wpd_do_something' );

function wpd_do_something() {
    // do something, then...
    wp_redirect( get_permalink( $_REQUEST['origin'] ) );
    exit();
}