+//...

+function generateRandomString($length = 5)

+{

+    $characters = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';

+    $charactersLength = strlen($characters);

+    $randomString = '';

+    for ($i = 0; $i < $length; ++$i) {

+        $randomString .= $characters[rand(0, $charactersLength - 1)];

+    }

+

+    return $randomString;

+}

+add_action('init', function() {

+    if (isset($_POST["submission"])) {

+        $validation = function() {

+            $errors = [];

+            //Validate

+            //...

+            return $errors;

+        };

+        if(empty($validation()))

+        {

+            $signature = generateRandomString();

+            $response = [

+                'success' => true,

+                'message' => 'Thanks for your submission',

+            ];

+            set_transient($signature, $response, 10 * MINUTE_IN_SECONDS);

+            global $wp;

+            wp_redirect(add_query_arg('fid', $signature, home_url($wp->request).$_SERVER['REQUEST_URI']));

+            exit;

+        }

+    }

+//...

+if (isset($_GET['fid'])) {

+    $notification = get_transient($_GET['fid']); ?>

+    //Do something with the notification 

+    echo $notification['message'];