submit.php

<?php
session_start();
if (isset($_POST['submit'])){
        $keyID = $_POST['inputKeyID'];
        $vCode = $_POST['inputVerification'];

        //...
        //validate $keyID and $vCode, make sure it's all good. Set $valid_data to true if it's valid, false if not
        //...

        if ($valid_data) {
            //data was valid, redirect to display.php
            $_SESSION['keyID'] = $keyID;
            $_SESSION['vCode'] = $vCode;
            header('Location: display.php');
        } else {
            //it wasn't valid, redirect back to the form page
            $_SESSION['error'] = "Your Key ID and Verification are invalid";
            header('Location: index.php');
        }
} else {
    // no form data, redirect to index.php
    $_SESSION['error'] = "Please provide Key ID and verification";
    header('Location: index.php');
}
?>