If you don’t mind exposing data too much you could use https://webhook.site/ as a testing target to check if any data is really sent. Alternatively you could run something like ‘nc -l -p $port’ on the server where you are hosting the php script and then use http://$server:$port as hook target (with $port being an unused port number >1024 and $server being the FQDN of the server hosting the PHP script). That will give you the raw http request.