One thing mentioned by Leon, that might not leap off the page at you, is that Zend Opcache is running for the WHOLE server. In other words, my main server has eight Moodle sites, plus some other types of sites running on it. Zend Opcache is taking care of ALL of them as shown by my monitoring script. This means that depending what you have on your server, 128M memory and 8000 for opcache.max_accelerated_files, might not be the optimum sizes to use. You might need to make both number bigger. And, it definitely pays to use one of the monitoring scripts that Ken provides the link to, so you can see at a glance what's going on.
I have one archived sever set to 12,000 files and when I turn it on and let it run a few hours for various types of testing, it winds up caching over 11,000 files, as it has every Moodle from 1.9 to 3.3 installed on it.