I just checked the code for PHP 7.1 and in there openssl_free_key is an alias for opnessl_pkey_free (PHP_FALIAS).
This supports Richards assumption.
I would also suggest to not use the alias,
but use the openssl_pkey_free function in new code.
This is because it is more descriptive about what its intend is and is the same structure as similar function in the openssl section.