Description:
------------
Hello,

I have came accross this issue after we have recieved random complains for 500ISE errors, which are solved after increasing  opcache.interned_strings_buffer value.

Through series of tests, it seems that for certain combination of values for opcache.interned_strings_buffer and opcache.memory_consumption can lead to error:
Fatal Error Zend OPcache cannot allocate buffer for interned strings.

I am reproducing it by this formul ( X is 8, 16, 32 etc. ):
 opcache.memory_consumption = X
 opcache.interned_strings_buffer = (X*7/8)/2

For every value for opcache.interned_strings_buffer >= (X*7/8)/2 the above error is produced.

Tested on PHP versions 7.0.22 and 7.1.8, Apache/2.4.27

Regards,
Ilian.