Set client debugging callback.
-> ***NOTE:***
+> ***NOTE:***
> This method has been added in v2.6.0, resp. v3.1.0.
## Params:
?>
-> ***NOTE:***
+> ***NOTE:***
> Except for http\Client::DEBUG_INFO, which always occurs separately, the debug
> callback's $type argument contains a bitmask of (http\Client::DEBUG_IN or http\Client::DEBUG_OUT)
> and (http\Client::DEBUG_HEADER or http\Client::DEBUG_BODY or http\Client::DEBUG_SSL).