php internals - What is the substitute for is_ref__gc in php7

In php5, zval struct has a member 'is_ref__gc'. We can use it as follow:

zval *x
...
...
if (!x->is_ref__gc)

   {
       printf("newx or newy isn't by ref");
       return;
   }  

However, in php7, zval doesn't have such a member.
What is the substitute for is_ref__gc in php7? i.e. how does I modify the above code?