Weak References allow the programmer to retain a reference to an object which does not prevent the object from being destroyed; They are useful for implementing cache like structures. They are currently supported in PHP by extension.

The current implementations of WeakRef all work by overloading object handlers, the most popular implementation (pecl-weakref) changes stack allocated read only memory and will be rendered unusable by PHP 7.3. Other implementations that change object handlers in some other way also risk faulting and rely on undefined behaviour and casting away constness.

Other implementations are possible, krakjoe/uref is an implementation that uses low level features (mprotect, 0xCC, signals) to implement weakrefs, but at the cost of portability, generating segfaults, protecting memory, and implicit signal handler restrictions.

In principle weak referencing an object is not complicated, and only requires the (ab)use of Zend or layers beneath because we do not directly support it.