Imagine you have a site API that accepts data in the form of GET requests with parameters, or as POST requests (say, with standard url-encoded, &-separated POST data).
You can use [ModSecurity][1] to view POST data.
Install on Debian/Ubuntu:
$ sudo apt install libapache2-mod-security2
Use the recommended configuration file:
$ sudo mv /etc/modsecurity/modsecurity.conf-recommended /etc/modsecurity/modsecurity.conf
Reload Apache:
$ sudo service apache2 reload
You will now find your data logged under /var/log/apache2/modsec_audit.log
$ tail -f /var/log/apache2/modsec_audit.log
--2222229-A--
[23/Nov/2017:11:36:35 +0000]
--2222229-B--
POST / HTTP/1.1
Content-Type: application/json
User-Agent: curl
Host: example.com
--2222229-C--
{"test":"modsecurity"}