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Sooner or later, you may need to assign the contents of one zval container to another. This is easier said than done, since the zval container doesn't contain only type information, but also references to places in Zend's internal data. For example, depending on their size, arrays and objects may be nested with lots of hash table entries. By assigning one zval to another, you avoid duplicating the hash table entries, using only a reference to them (at most).
To copy this complex kind of data, use the copy constructor. Copy constructors are typically defined in languages that support operator overloading, with the express purpose of copying complex types. If you define an object in such a language, you have the possibility of overloading the "=" operator, which is usually responsible for assigning the contents of the lvalue (result of the evaluation of the left side of the operator) to the rvalue (same for the right side).
Overloading means assigning a different meaning to this operator, and is usually used to assign a function call to an operator. Whenever this operator would be used on such an object in a program, this function would be called with the lvalue and rvalue as parameters. Equipped with that information, it can perform the operation it intends the "=" operator to have (usually an extended form of copying).
This same form of "extended copying" is also necessary for PHP's zval containers. Again, in the case of an array, this extended copying would imply re-creation of all hash table entries relating to this array. For strings, proper memory allocation would have to be assured, and so on.
Zend ships with such a function, called zend_copy_ctor() (the previous PHP equivalent was pval_copy_constructor()).
A most useful demonstration is a function that accepts a complex type as argument, modifies it, and then returns the argument:
The first part of the function is plain-vanilla argument retrieval. After the (left out) modifications, however, it gets interesting: The container of parameter is assigned to the (predefined) return_value container. Now, in order to effectively duplicate its contents, the copy constructor is called. The copy constructor works directly with the supplied argument, and the standard return values are FAILURE on failure and SUCCESS on success.
If you omit the call to the copy constructor in this example, both parameter and return_value would point to the same internal data, meaning that return_value would be an illegal additional reference to the same data structures. Whenever changes occurred in the data that parameter points to, return_value might be affected. Thus, in order to create separate copies, the copy constructor must be used.
The copy constructor's counterpart in the Zend API, the destructor zval_dtor(), does the opposite of the constructor.