Zend OP Cache is key in having a very high performance Software Application running in PHP.
A good way to see how it is working, which files are cached and more, is using this very nice PHP Script.
copy ocp.php from my github repository to the folder where your virtual host configuration file is pointing for the index.php
$ nano /var/www/cool_project/public/ocp.php
Click on the copy raw icon and paste the content of the file.
Save the file and if you set any of the virtual hosts configuration files posted here, you just need to open https://your.project.domain/ocp.php and you will see something like this:
Click on the Files button link and click on ungroup to get a list of all cached files and some useful stats: