The bad:
"Use HHVM". Do not do this, just upgrade to PHP 7. HHVM does have better performance [in some cases, not all], but it also has many unsupported modules [such as Memcache based sessions]. It's difficult to get improvements added and you're going to get frustrated by something people in the mainsteam PHP branch can do that you cannot do.
?int $var = null;
This is a question of style. Personally, i can read it, but it is a little odd and I've seen people get confused over it. I'd personally avoid it. The null check and type check is much clearer.