The best configuration really depends on what type of site you're running. Is this strictly a PHP site or is it database driven (i.e. WordPress, Joomla, Drupal, etc), or something else?
From the CLI, can you run
top then hit
c, and finally
e and post screenshot link so we can take a closer look at some general stats?
The arguments mentioned after
top simply sort the process list and make it easier to read.
Also, when things begin to die, are you seeing anything logged? If you would, please post the output of:
tail -20 /var/log/nginx/error.log
/var/log you should also see a log for your specific PHP version. If you'll run the same command as above for that log file as well, that'll let us take a closer look at what's happening with PHP as well.