email-templates/public/email.php

<?php

$slug = $_POST['slug'];
$topic = $_POST['topic'];
$emails_json = json_decode(file_get_contents(__DIR__.'/emails.json'), true)[0];
if (!array_key_exists($topic, $emails_json)) {
    header('HTTP/1.0 404 Not Found');
}
$emails_json = $emails_json[$topic];
$emails = [];
foreach ($emails_json as $email) {
    $emails[$email['slug']] = $email;
}

if (!array_key_exists($slug, $emails)) {
    header('HTTP/1.0 404 Not Found');
}

$email = $emails[$slug];

header('Content-Type: application/json');
die(json_encode($email));