php-oath | PHP HOTP/TOTP Implementation | Authentication library

I'll try to sketch our situation:

• We have a Wordpress website that needs to connect to an Oath2 API
• I have little experience yet using API's and PHP
• We are using a PHP-Oath2 library, which can be found here: https://github.com/adoy/PHP-OAuth2/tree/master/src/OAuth2

The method from the libary where we're stuck is executeRequest. This method can be found on line 404 (oh the irony) of https://github.com/adoy/PHP-OAuth2/blob/master/src/OAuth2/Client.php

Now, like I said, I've little experience in PHP so my way of debugging is just echo'ing the status of variables at given points in the script. Amateuristic as this may be, I could at least verify that the url and parameters are correct. I could print the complete URL (dummy parameter values ofc):

Mind the following parameters used: code, redirect_uri, grant_typen client_id and client_Secret.

https://oauth.smartschool.be/OAuth/index/token?code=irsyB0VSmg4V5dVjHFgdl85iRvvu3gYpsuIE4cOk&redirect_uri=https%3A%2F%2Fmycallbackurl.com%2Fmy-page%2F&grant_type=authorization_code&client_id=5d6t5ev5a6d8&client_secret=pada9c54a6sc

When you execute this request, you'll get a JSON error message. Which is normal, as I cannot give you our client_id and client_secret: