Make a simple CashBook entry using SOAP and PHP

I would like to make a simple cashbook entry everytime a purcahse is made on our platform. The entry should contain:

• Amount
• Account number (e.g. 5820)
• (optionally) some text

I have found a PHP Economic SOAP library and struggled with it for some hours, to then figure out that it does not seem to be able to make a cashbook entry: https://github.com/xown/economic-soap-php-client

Can someone help me out with a snippet or something that can solve this for me? I am lost...

Update:

Soldering on, I have tried to extend the CashBookService in the Economic SOAP PHP Client (https://github.com/xown/economic-soap-php-client).

I have created a new method in the model like this:

And then trying to call it like this: