WeakMap polyfill for PHP 7.4

This polyfill aims to be 100% compatible with WeakMap in PHP 8.

Introduction

PHP 7.4 introduced WeakReference, but didn't include a WeakMap implementation. This has been implemented since, but is only available in PHP 8.

The RFC author, Nikita Popov, highlights why a userland WeakMap is suboptimal:

Weak maps require first-class language support and cannot be implemented using existing functionality provided by PHP.

At first sight, it may seem that an array mapping from spl_object_id() to arbitrary values could serve the purpose of a weak map. This is not the case for multiple reasons:

• spl_object_id() values are reused after the object is destroyed. Two different objects can have the same object ID – just not at the same time.
• The object ID cannot be converted back into an object, so iteration over the map is not possible.
• The value stored under the ID will not be released when the object is destroyed.

Using the WeakReference class introduced in PHP 7.4, it is possible to avoid the first two issues (…). However, this does not solve the third problem: The data will not be released when the object is destroyed. It will only be released on the next access with an object that has the same reused ID, or if a garbage collection mechanism, which performs regular sweeps of the whole map, is implemented.

A native weak map implementation will instead remove the value from the weak map as soon as the object key is destroyed.

This is the trade-off this library offers: a 100% compatible implementation, but:

• slower
• whose values are not removed as soon as the object key is destroyed, but when you use the WeakMap again; note that this affects when object destructors are called as well

Here is how it works:

• calls to count() will always garbage collect dangling references immediately
• using array-like features: set, get, isset(), unset() will perform garbage collection at least every 100 operations (less often for large WeakMaps).
• when GC cycles are collected (automatically or via gc_collect_cycles()), garbage collection will trigger immediately

This offers a reasonable trade-off between performance and memory usage.

Installation

This library is installable via Composer:

composer require benmorel/weakmap-polyfill

Requirements

This library requires PHP 7.4 or later.

Quickstart

$weakMap = new WeakMap();

$a = new stdClass();
$b = new stdClass();

$weakMap[$a] = 123;

var_export(isset($weakMap[$a])); // true
var_export(isset($weakMap[$b])); // false

echo $weakMap[$a]; // 123
echo $weakMap[$b]; // Error

echo count($weakMap); // 1

// removing the last reference to the object will remove it from the WeakMap
unset($a);

echo count($weakMap); // 0

Alternatives

The weakreference_bc PECL backports a native polyfill for WeakReference and WeakMap to PHP 7.0-7.4. The PECL has the following advantages:

• weakreference_bc supports PHP 7.0+
• Like the real WeakMap/WeakReference, values are removed as soon as the object key is destroyed.
• Native implementations are faster and use less memory.
• weakreference_bc's WeakMap::count() is fast because it does not need to perform garbage collection.

This has the following drawbacks:

• Installing a PECL has more steps than installing a composer package.
• weakreference_bc currently implements Iterator instead of the more accurate IteratorAggregate as of 0.4.1.