Debug application response?

Question

css

I'm trying to connect to the site Interkasu to receive payment. In the settings, ask to specify certain URLs

. That is, after the payment is made, the service will send a specific request to these URLs with specific data.
The question is how to debug it?
I thought maybe with the help of some online http server such as httpbin.org , but requests are sent exclusively to the pages of the site that has already been added.
That is, I added my site mysite.com and now I need to somehow debug what comes to the page mysite.com/succes.php I
thought about xdegub but it is not supported on my hosting.
Please tell me how can I debug responses from the service on a local or remote machine?
Found something like this
file_put_contents( 'debug' . time() . '.log', var_export( $_POST, true));
Can anyone else come up with options?
PS and the joke is that I don’t pull the service and get a response, but that this service will pull a certain URL on my site

Answer 1

text-align: center;
overflow: hidden;
line-height: 1.5;
text-overflow: ellipsis;
white-space: nowrap;

Answer 2

Interkassa shows that your site has responded to the URL interaction request.

Answer 3

Rent a VPS, you can install xdebug or a sniffer.