Namecheap Synology DSM DDNS provider

ddns_provider.conf

#Insert this at the end of /etc.defaults/ddns_provider.conf
[Namecheap]
        modulepath=/usr/syno/bin/ddns/namecheap.php
        queryurl=https://dynamicdns.park-your-domain.com/update

namecheap.php

#!/usr/bin/php -d open_basedir=/usr/syno/bin/ddns
<?php

if ($argc !== 5) {
  echo 'badparam';
  exit();
}

$account = $argv[1];
$pwd = (string)$argv[2];
$hostname = (string)$argv[3];
$ip = (string)$argv[4];

// check the hostname contains '.'
if (strpos($hostname, '.') === false) {
  echo 'badparam';
  exit();
}

// only for IPv4 format
if (!filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4)) {
  echo "badparam";
  exit();
}

$url = 'https://dynamicdns.park-your-domain.com/update?host='.$account.'&domain='.$hostname.'&password='.$pwd.'&ip='.$ip;

$req = curl_init();
curl_setopt($req, CURLOPT_URL, $url);
curl_setopt($req, CURLOPT_RETURNTRANSFER, true);
$badXmlString = curl_exec($req); // XML String returned has a wrong xml utf declaration
curl_close($req);

// Heading says content is UTF-16, but it is UTF-8, so it is manually changed here
$fixedXmlString = preg_replace('/(<\?xml[^?]+?)utf-16/i', '$1utf-8', $badXmlString);
$xml = new SimpleXMLElement($fixedXmlString);
if ($xml->ErrCount > 0) {
  $error = $xml->errors[0]->Err1;
  if (strcmp($error, "Domain name not found") === 0) {
    echo "nohost";
  } elseif (strcmp($error, "Passwords do not match") === 0) {
    echo "badauth";
  } elseif (strcmp($error, "No Records updated. A record not Found;") === 0) {
    echo "nohost";
  } else {
    echo "911 [".$error."]";
  }
} else {
  echo "good";
}

The script at the top of this gist (which now contains the modifications from @bloedboemmel) is working for me.
Couple of things that I found helpful:

1. Reading the official Namecheap documentation about performing the HTTP GET call to the URL: https://www.namecheap.com/support/knowledgebase/article.aspx/29/11/how-to-dynamically-update-the-hosts-ip-with-an-http-request/
2. The detailed instructions from @Dade-R which show the crucial step of adding the executable permission to the PHP file.

Great stuff, and thanks to all involved!