$error messate POST into div in index.php

This is my first question on this site however I have been here millions of times looking for answer and I hope you can help me out this time as I wasn't able to figure it out myself.

I am not very skilled when it comes to coding but was hoping to amend the php contact form I have found online (it's free), the form itself is worki9ng but what I am struggling with is to set the errors to be printed on the main page where the form is, at them moment the yare displayed on the contact.php form which is where the php code is and I would like it all displayed in a div for example on index.php just under the form.

my contact.php

<?php
if (isset($_POST['submit'])) {}
    
$sendTo = "email address";
$subject = "msg content";
$headers = 'From: email address' . "\r\n";
    
$name = @$_POST['name'];
$email = @$_POST['email'];
$message = @$_POST['message'];
        
$okMessage = 'thank you';  
$errorMessage = 'Please fill all the fields';
$url = 'https://www.google.com/recaptcha/api/siteverify';
$privatekey = "----------------------------------------"; 
$response = file_get_contents($url."?secret=".$privatekey."&response=".$_POST['g-recaptcha-response']."&remoteip=".$_SERVER['REMOTE_ADDR']);
$data = json_decode($response);
    
$emailText = "Name: $name \n Email: $email \n Message: $message";
    
if (isset($data->success) AND $data->success==true) {
    
    mail($sendTo, $subject, $emailText, $headers);
    $responseArray = $okMessage;
}
else
{
    //verification failed
    $responseArray = $errorMessage;
}
    
if (!empty($_SERVER['HTTP_X_REQUESTED_WITH']) && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest') {
    $encoded = json_encode($responseArray);
    
    header('Content-Type: application/json');
    
    echo $encoded;
}
else {
    echo $responseArray;
}
?>

<form method="post" name="contactform" action="contact.php" >
                                    <div class="field half first">
                                        <label for="name">name</label>
                                        <input type="text" name="name" id="name" placeholder="name" />
                                        
                                    </div>
                                    <div class="field half">
                                        <label for="email">Email</label>
                                        <input type="text" name="email" id="email" placeholder="email@email.com" />
                                    </div>
                                    <div class="field">
                                        <label for="message">message</label>
                                        <textarea name="message" id="msg" rows="4" placeholder="type your message here"></textarea>
                                    </div>
                                    <div class="g-recaptcha" data-sitekey="----------------------------------"></div>
                                    
                                    <ul class="actions">
                                        <li><input type="submit" value="send" class="special" /></li>
                                        <li><input type="reset" value="reset pola" /></li>
                                    </ul>

I have tried to use $_POST and $_GET to echo responseArray but I can't figure it out...

Is anyone able to provide a code I would have to use to display $okMessage and $errorMessage on index.php please?

Many thanks for all help