Get data from a callback in php

Hi

I am struggling with how to get in php the data attached to a callback from a site that uses Swagger
I have tried :

* json_encode($_POST) 

{"data":"state","eventType":"Payment"}

almost nothing useful

* $json = file_get_contents('php://input');

It is a strange response
"data=payer&data=items&data=payments&data=amount&data=id&data=date&data=formSlug&data=formType&data=organizationSlug&eventType=Order"
Where are the data ?
What is the best way to get the data, in a php callback page ?

Thanks