Recent versions of PHP have a cache of filenames for knowing the real path of files, and require_once() and include_once() can take advantage of it.
There's a value you can set in your php.ini to set the size of the cache, but I have no idea how to tell what the size should be. The default value is 16k, but I see no way of telling how much of that cache we're using. The docs are vague:
Yes, I can jack up the amount of cache allowed, and run tests with ab or some other testing, but I'd like something with a little more introspection than just timing from a distance.
You've probably already found this, but for those who come across this question, you can use realpath_cache_size() and realpath_cache_get() to figure out how much of the realpath cache is being used on your site and tune the settings accordingly.
Though I can't offer anything specific to your situation, my understanding is that 16k is pretty low for most larger PHP applications (particularly ones that use a framework like the Zend Framework). I'd say at least double the cache size if your application uses lots of includes and see where to go from there. You might also want to increase the TTL as long as your directory structure is pretty consistent.
the 16K is the # of files not activity.
Set to 1k for most sites. Very similar to settings in APC, xcache ea etc.
来源：https://stackoverflow.com/questions/197734/how-can-i-tune-the-php-realpath-cache