Modify the ht of ZVAL?

Hi all, I'm moving onto zval part of my studying now. Just wondering,
how a zval in another zval can modified.

I've been reading the documents, and found that zval is normally
forbidden from modifying if not passed by reference, and can be
separated manually. Using hash* functions allows adding content to a
zval (key=>value pair of an associated array for instance).

However, how can it be modified? Would add_assoc_* overwrite a key with
new value? Or does hash_* functions? Or should I just create another
empty zval and copy everything their with a filter?

Thanks!