HTML5 form with PHP action on Heroku - php

Question

I have a contact form on my site that is hooked up to a Mailgun server to send email upon submit.
All is working fine locally, but when hosted on Heroku, the form doesn't act as expected and instead triggers a download. Any idea what needs to be done to get this to behave properly?
I've attempted making my index.html an index.php and putting the PHP script in <?php ?> tags at the top of the page - both of which worked locally - but neither has worked on Heroku.
Thank you!
form from index.html:
<form method="post" name="contact_form" action="form.php">
<h1>want to chat?</h1>
<input type="text" name="name" placeholder="name" required>
<input type="email" name="email" placeholder="email" required>
<textarea name="message" cols="30" rows="10" placeholder="drop a message..." required></textarea>
<input type="submit" value="send off" name="submit">
<h1 class="email-thanks"></h1>
</form>
form.php file:
<?php
require '../vendor/autoload.php';
use Mailgun\Mailgun;
session_start();
if(isset($_POST['submit'])){
$name = $_POST['name'];
$email = $_POST['email'];
$message = $_POST['message'];
# Instantiate the client.
$mgClient = new Mailgun('key-xxxxxxxx');
$domain = "mail.laurenfazah.com";
# Make the call to the client.
$result = $mgClient->sendMessage($domain, array(
'from' => $name . ' <' . $email . '>',
'to' => 'Lauren <example#gmail.com>',
'subject' => 'Portfolio Message',
'text' => $message
));
}
header( 'Location: /' ) ;
session_destroy();
?>

Answer 1

Sounds like the server it is hosted on doesn't have PHP installed.