Explain PHP's garbage collection mechanism. How does it handle circular references and what are the performance implications?

PHP Garbage Collection Mechanism:

Reference Counting + Cycle Collection:

// Example of circular reference
class Node {
    public $data;
    public $parent;
    public $children = [];
    
    public function addChild(Node $child): void {
        $this->children[] = $child;
        $child->parent = $this; // Creates circular reference
    }
}

$root = new Node();
$child = new Node();
$root->addChild($child); // Circular reference created

// Without proper cleanup, this creates memory leak
unset($root, $child); // Variables removed but objects still referenced

How PHP Handles This:

1. Reference Counting: Each zval has refcount
2. Cycle Detection: When refcount decreases, check for cycles
3. Mark & Sweep: Mark reachable objects, sweep unreachable ones

Garbage Collection Process:

// Manual GC control
gc_enable();           // Enable garbage collection
gc_collect_cycles();   // Force garbage collection
gc_disable();          // Disable garbage collection

// Monitor GC performance
$before = gc_status();
// ... your code ...
$after = gc_status();

echo "Cycles collected: " . ($after["collected"] - $before["collected"]);
echo "Memory freed: " . ($before["memory"] - $after["memory"]) . " bytes";

Performance Implications:

• CPU Overhead: GC runs periodically, can cause pauses
• Memory Overhead: Additional metadata for cycle detection
• Threshold Based: GC triggers when root buffer fills (10,000 items)

Optimization Strategies:

// 1. Explicit cleanup for circular references
class Node {
    public function cleanup(): void {
        foreach($this->children as $child) {
            $child->parent = null;
            $child->cleanup();
        }
        $this->children = [];
    }
}

// 2. Weak references (PHP 7.4+)
class Parent {
    private WeakMap $children;
    
    public function __construct() {
        $this->children = new WeakMap();
    }
    
    public function addChild(Child $child): void {
        $this->children[$child] = true;
        // No strong reference, no circular dependency
    }
}

// 3. Use SplObjectStorage for object collections
$storage = new SplObjectStorage();
$storage->attach($object); // Weak reference-like behavior

Best Practices:

• Avoid circular references when possible
• Use weak references for parent-child relationships
• Monitor memory usage in long-running processes
• Consider manual gc_collect_cycles() in memory-intensive operations