Timing attack safe string comparisonCompares two strings using the same time whether they're equal or not.
Note: It can leak the length of a string when arguments of differing length are supplied.
This function was added in PHP 5.6.
However, the Hash extension may be explicitly disabled on select servers.
As of PHP 7.4.0, the Hash extension is a core PHP extension and can no
longer be disabled.
I.e. when PHP 7.4.0 becomes the minimum requirement, this polyfill
can be safely removed.