[CORE] Use random_bytes() if available and improve common_confirmation_code() randomness.

[CORE] Use random_bytes() if available and improve common_confirmation_code() randomness.
With PHP 7 comes the [random_bytes()](https://php.net/manual/en/function.random-bytes.php) and the [random_int()](https://www.php.net/manual/en/function.random-int.php) function which generates cryptographically secure pseudo-random bytes and integers, respectively.