Low Opcache Interned Strings Storage
The Opcache extension can cache so called interned PHP strings in shared memory, so that the memory is allocated only once for these strings across all scripts. This reduces the memory overhead of every request.
Interned strings are for example literal strings inside your scripts like "Hello World" in this example:
echo "Hello World";
There is other data represented as interned strings inside the PHP engine such as class names, file names, property names and many other things.
This cache should never be maxed out, because its an easy performance and
memory win for every application. That is why its important to monitor that the
default of 8 MB for
opcache.interned_strings_buffer is large enough.
Increase this setting to a higher value if it runs low.
But don’t just increase the value to a much higher value than needed, because the whole specified memory is allocated by Opcache.
Manual: Opcache Configuration