WeakMap (PHP 8.0)

Also Known As

TL;DR

Explanation

A WeakMap holds object keys weakly — when the only remaining reference to a key object is in the WeakMap, that object is garbage collected and its WeakMap entry is automatically removed. This contrasts with SplObjectStorage and arrays, which hold strong references and prevent GC. Typical use cases: per-object caches (memoising computed values on an entity without modifying the entity class), attribute caches in ORMs, and proxy metadata storage. WeakMap is iterable and Countable. PHP 8.0+; the RFC was specifically motivated by Doctrine's need to associate metadata with entities without preventing their deallocation.

Common Misconception

Why It Matters

Common Mistakes

• Using a regular array or SplObjectStorage for object-keyed caches — holds strong references, preventing GC.
• Not checking if a WeakMap key still exists before access — the object may have been collected.
• Using WeakMap for non-object keys — it only accepts objects as keys.
• Not realising WeakMap is not iterable when the GC runs between iterations — do not iterate a WeakMap.

Code Examples

// Regular array cache — prevents GC of $user objects:
class Cache {
    private array $data = [];
    public function set(object $key, mixed $val): void {
        $this->data[spl_object_id($key)] = $val; // Strong reference via ID
    }
}

// WeakMap — GC can collect $user when no other references exist:
$cache = new WeakMap();
$cache[$user] = computeExpensiveData($user); // Freed when $user is GC'd

$cache = new WeakMap();
function expensiveData(object $obj, WeakMap $cache): array {
    return $cache[$obj] ??= computeExpensiveData($obj);
    // Entry removed automatically when $obj is garbage collected
}

References

• ↗ https://www.php.net/manual/en/class.weakmap.php

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