PHP 7.3 support

[pinepain]

From @krakjoe feedback in SO PHP chat, let's give it a look as this change in php-src may break things in weakref PHP extension and we may be also affected.

[pinepain]

I think Weak References RFC is far superior way to solve this type of problem and more secure. As I don't have plan to maintain php-ref, I think those who really need weak references in PHP to look into https://github.com/colder/php-weakref and https://github.com/krakjoe/uref or reconsider whether you approach fits PHP design or whether PHP design fits your problem.