cc @bukka

Unfortunately this is no longer supported in OpenSSL 1.1 so I'm not too keen to add just for 1.0 as it will create difference that we can't fix. Please see

https://github.com/openssl/openssl/blob/e2cefab06a9e1b8d9a21d030754f62dfbb199950/crypto/dh/dh_lib.c#L251

(you can't supply an empty public key).

The DH part won't set the key too in OpenSSL 1.1 for the the similar reason. So it would again lead to a different result.

Yeah, that is really good point. I didn't realize it would not work in OpenSSL 1.1. I'm going to close this PR. Thanks

@bukka this will introduce a BC break for people using PHP 7.1 and OpenSSL 1.0.x. For instance, Ubuntu 16 offers OpenSSL 1.0.2g as default. I think we should trigger a warning or a deprecation notice if someone will pass a priv_key in dh parameter for openssl_pkey_new(). Basically, the Diffie Hellman keys can be generated only with g (generator) and p (prime) parameters, starting from PHP 7.1.

@ezimuel I put together #2208 which seems to work fine with OpenSSL 1.1 but needs more testing on lower versions.