First, working version...

Usage:
php ./update.php host example.com

.gitignore

+config.php

config.php.sample

+<?php
+$soap_location = 'https://ispconfig.example.com:8080/remote/index.php';
+$soap_uri = 'https://ispconfig.example.com:8080/remote/';
+$soap_user = 'username';
+$soap_password = 'password';
+

update.php

+<?php
+require('config.php');
+
+if ($argc<3) {
+	print "Usage: php ./update.php HOST DOMAIN [IP]\n";
+	print "If not given, the ip address will be queried from icanhazip.com\n";
+	die();
+}
+
+$ddns_host = $argv[1];
+$domain = $argv[2];
+if ($argc>3) {
+	$ip = $argv[3];
+} else {
+	// Figure out the public IP of this host:
+	$ip = trim(file_get_contents("http://icanhazip.com/"));
+	if (filter_var($ip, FILTER_VALIDATE_IP) === false) {
+		die("Unable to retrieve public IP address (icanhazip.com returned $ip)\n");
+	}
+}
+
+// print("Setting DDNS host $ddns_host.$domain to IP $ip\n");
+
+$client = new SoapClient(null,
+      array('location'   => $soap_location,
+            'uri'        => $soap_uri,
+            'trace'      => 1,
+            'exceptions' => 1));
+
+try {
+        $session_id = $client->login($soap_user, $soap_password);
+
+        $zone_id = $client->dns_zone_get_id($session_id, $domain);
+        $zone = $client->dns_zone_get($session_id, $zone_id);
+        $records = $client->dns_rr_get_all_by_zone($session_id, $zone_id);
+        
+        $dns_record = null;
+        foreach ($records as $rec) {
+            if ($rec['type']=='A' && $rec['name']==$ddns_host) {
+                $dns_record = $rec;
+            }
+        }
+        if (is_null($dns_record)) {
+            $client->logout($session_id);
+            die("Unable to find DNS record for host $ddns_host in domain $domain on the server...\n");
+        }
+        if ($dns_record['data'] != $ip) {
+			$dns_record['data'] = $ip;
+			$dns_record['serial'] = $dns_record['serial']+1;
+			$client->dns_a_update($session_id, null, $dns_record['id'], $dns_record);
+ 
+			$zone['serial'] = $zone['serial'] + 1;
+			$client->dns_zone_update($session_id, 0, $zone_id, $zone);
+
+			print("Successfully set DNS entry for host $ddns_host in domain $domain to $ip.\n");
+		} else {
+// 			print("IP address of $ddns_host.$domain already set to $ip.\n");
+		}
+
+        $client->logout($session_id);
+ 
+} catch (SoapFault $e) {
+    die('SOAP Error: '.$e->getMessage()."\n");
+}
+ 
+?>
\ No newline at end of file