I used this as my source:
https://php-osx.liip.ch/ and followed the instructions there, installing from the terminal using sudo. I also updated the root's .profile as instructed in the first of the Frequently asked Questions, but I'm not sure if that is necessary, probably needed only for command line use by someone who has sudoed to root.
CPU usage is 100%; RAM usage (4GB) isn't
Does this mean that you have 4GB or ram but it is not 100% in use? That could be a possible source of the problem since the standard mac mini server comes with 8GB and it is not only physical memory that is involved but the swap. If for some reason there is a great deal of swapping back and forth, that could become a problem. My macbook pro, just idling, uses more than 4GB of physical memory. And I would assume that the server is even more memory inntensive.
One possibility is that you have a plugin which is eating up memory and cpu because of some incompatiblity. It is usually a good idea to disable your plugins and reenable themn one at a time.
The web server's error log is found in /var/log/apache2.