<?php

$secret_key = 'CHANGE-THIS';
$present_file = 'present.json';

// check for POST request
if ($_SERVER['REQUEST_METHOD'] != 'POST') {
    error_log('FAILED - not POST - ' . $_SERVER['REQUEST_METHOD']);
    http_response_code(405);
    exit();
}

// get content type
$content_type = isset($_SERVER['CONTENT_TYPE']) ? strtolower(trim($_SERVER['CONTENT_TYPE'])) : '';

if ($content_type != 'application/json') {
    error_log('FAILED - not application/json - ' . $content_type);
    http_response_code(400);
    exit();
}

// get payload
$payload = trim(file_get_contents("php://input"));

if (empty($payload)) {
    error_log('FAILED - no payload');
    http_response_code(400);
    exit();
}

// get header signature
$header_signature = isset($_SERVER['HTTP_X_HMAC_HASH']) ? $_SERVER['HTTP_X_HMAC_HASH'] : '';
if (empty($header_signature)) {
    error_log('FAILED - header signature missing');
    http_response_code(401);
    exit();
}

// calculate payload signature
$payload_signature = hash_hmac('sha256', $payload, $secret_key, false);

// check payload signature against header signature
if ($header_signature !== $payload_signature) {
    error_log('FAILED - payload signature');
    http_response_code(401);
    exit();
}

// convert json to array
$decoded = json_decode($payload, true);

// check for json decode errors
if (json_last_error() !== JSON_ERROR_NONE) {
    error_log('FAILED - json decode - ' . json_last_error());
    http_response_code(400);
    exit();
}

// success, do something
http_response_code(200);

$present = array(
    "timestamp" => time(),
    "names" => sort($decoded)
);

file_put_contents($present_file, json_encode($present));