How avoid form resubmission on page refresh? - wordpress

Question

I'm doing a simple feedback form on WordPress. And like many people, I encountered the problem of resending the form when refresh the browser page. I know that this problem is solved through the use of the pattern "Post/Redirect/Get". Which says that you need after processing the data $_POST, request the same page using the $_GET method. But I can not use the result of the wp_mail function for redirection.
if(wp_mail($email, $email_subject, $email_message, $headers)) {
add_action('send_headers', 'simplemail_add_header');
}
function simplemail_add_header() {
header("Location: http://google.com");
}
It just does not work.
UPD
Here is my full code:
class SimpleMailer {
private $nonce = 'feedback_nonce';
public function __construct() {
add_action('phpmailer_init', array($this, 'simplemail_smtp_config'));
add_shortcode('simplemail', array($this, 'simplemail_sendmail'));
}
public function simplemail_smtp_config($phpmailer) {
$phpmailer->isSMTP();
$phpmailer->SetFrom("admin#mail.com");
$phpmailer->addAddress("sender#mail.com");
$phpmailer->Host = "ssl://smtp.mail.com";
$phpmailer->SMTPAuth = true;
$phpmailer->Port = 465;
$phpmailer->Username = "admin#mail.com";
$phpmailer->Password = "password";
$phpmailer->SMTPSecure = 'ssl';
}
public function simplemail_sendmail($shortcode_attributes) {
global $wp;
$result = "";
$error = false;
$data = array();
$required_fields = array("feedback_name", "feedback_email", "feedback_message");
$atts = shortcode_atts(array(
"email" => get_bloginfo('admin_email'),
"form_action" => home_url($wp->request),
"form_cls" => '',
"mail_subject" => "Feedback message from",
"pls_name" => 'Your Name',
"pls_email" => 'Your E-mail Address',
"pls_message" => 'Your Message',
"label_submit" => 'Submit',
"error_common" => 'There was some mistake. Try again, a little later.',
"error_empty" => 'Please fill in all the required fields.',
"error_noemail" => 'Please enter a valid e-mail address.',
"success" => 'Thanks for your e-mail! We\'ll get back to you as soon as we can.'
), $shortcode_attributes);
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
foreach ($_POST as $field => $value) {
if (get_magic_quotes_gpc()) {
$value = stripslashes($value);
}
$data[$field] = trim(strip_tags($value));
}
foreach ($required_fields as $required_field) {
$value = trim($data[$required_field]);
if(empty($value)) {
$error = true;
$result = $atts['error_empty'];
}
}
if(!empty($data["feedback_blank"])) {
$error = true;
$result = $atts['error_empty'];
}
if(!is_email($data['feedback_email'])) {
$error = true;
$result = $atts['error_noemail'];
}
if(!wp_verify_nonce($data[$this->nonce],'simplemail_nonce')) {
$error = true;
$result = $atts['error_common'];
}
if ($error == false) {
$email_subject = $atts['mail_subject']." [".get_bloginfo('name')."]";
$email_message = $data['feedback_message']."\n\n";
$headers = "From: ".$data['feedback_name']." <".$data['feedback_email'].">\n";
$headers .= "Content-Type: text/plain; charset=UTF-8\n";
$headers .= "Content-Transfer-Encoding: 8bit\n";
if(wp_mail(null, $email_subject, $email_message, $headers)) {
add_action('send_headers', array($this, 'simplemail_add_header', 10, $atts['form_action']));
// wp_redirect( 'http://google.com', 301 );
// exit;
}
$data = array();
$result = $atts['success'];
}
}
return $this->simplemail_draw_form($atts, $data, $result);
}
public function simplemail_draw_form($atts, $data, $result) {
$output = "<form action='".$atts['form_action']."' class='".$atts['form_cls']."' method='post'>".PHP_EOL.
"<input type='text' name='feedback_name' placeholder='".$atts['pls_name']."' value='".#$data['feedback_name']."'>".PHP_EOL.
"<input type='text' name='feedback_blank'>".PHP_EOL.
"<input type='email' name='feedback_email' placeholder='".$atts['pls_email']."' value='".#$data['feedback_email']."'>".PHP_EOL.
"<textarea name='feedback_message' cols='30' rows='10' placeholder='".$atts['pls_message']."'>".#$data['feedback_message']."</textarea>".PHP_EOL;
$output .= wp_nonce_field('simplemail_nonce', $this->nonce, false);
$output .= ($result != "") ? '<div class="feedback-info">'.$result.'</div>' : '<div class="feedback-info"></div>';
$output .= "<button type='submit'>".$atts['label_submit']."</button>".PHP_EOL."</form>";
return $output;
}
public function simplemail_add_header($location) {
header("Location: {$location}");
}
}
$simplemailer = new SimpleMailer();
And I get this error if I uncomment the redirect. And nothing at all, if you try to use simplemail_add_header
Warning: Cannot modify header information - headers already sent by (output started at /var/www/vhosts/12/151953/webspace/httpdocs/skgk.kz/wp-includes/nav-menu-template.php:256) in /var/www/vhosts/12/151953/webspace/httpdocs/skgk.kz/wp-includes/pluggable.php on line 1216

Answer 1

I think you need to add a token in a hidden textbox and within the form to be submitted, the text in this text box will be the token and it need to change on every page load. Save this token in a session variable. Then add a condition at the top of the page to validate the token, if the token is different kill the loading process or display a message or whatever you feel is needed. You may also add token longevity to allow submitting of a page within certain amount of time.
The token creation, token validation and token longevity are normally a function somewhere that is called as needed and form different pages.
Edit:
If all you want is redirect the user to a different page then do:
if(mail succeed) {
header('location: thankyou.html');
}