How does the Zend Engine manage memory with Zvals, Reference Counting, and the Circular Garbage Collector in PHP 8.x?

Zval Structure & Reference Counting

In PHP, variables are stored in a zval (Zend Value) struct containing:

1. value: Union storing the primitive or pointer.
2. u1.type_info: Type identifier.
3. refcount: Number of variables pointing to this compound value (arrays, objects).

The Circular Reference Trap

When Object A points to Object B, and Object B points to Object A:

$a = new Node();
$b = new Node();
$a->sibling = $b;
$b->sibling = $a;

unset($a, $b); // Refcounts decrement from 2 to 1, but never reach 0!

Even after unset(), both objects remain in memory because their refcount is 1.

Solving Daemon Memory Leaks

1. PHP Cycle Collector Buffer: The GC buffer collects potential cyclic roots (10,000 max by default).
2. Explicit Garbage Collection Trigger:

   gc_collect_cycles(); // Forces cycle collection in loop
   gc_mem_caches();     // Releases cached memory chunks back to OS
   

3. Weak References (PHP 8.0+): Use WeakReference so child nodes do not increment the parent's refcount:

   $this->parent = WeakReference::create($parentNode);