modernisation of the code

I think you should make a Config class for parameter management that you could extend with an ArrayAccess to have a pseudo toArray(). 

Then you should put the equivalent of your http client in PSR-7 and PSR-18 so that the implementations / tests etc... are much simpler. 

Do you know what I mean? 

https://github.com/opencoconut/coconutphp/blob/master/src/Client.php#L14