The Nefit Easy uses XMPP to communicate with the server (Bosch) The Nefit Easy app uses SASL DIGEST-MD5 for authentication on the server. The server response is encrypted with AES-256/ECB/NoPadding. The key is generated using the access key (as mentioned on the Nefit Easy casing) and the user password.
If you want to run commands periodically (say, more than once every few minutes), you should consider running a HTTP server, which opens a connection to the backend at startup and reuses it for all commands sent to it.
For a PHP implementation you can use XMPPHP.