Weakmap

WeakMap is a special object type: it is a map, or key–value storage, where the keys are objects and the stored values are automatically removed when the object key is no longer referenced anywhere else in your program.

WeakMap may be considered as an associative array, where the keys must be objects. It also doesn’t prevent objects from being garbage-collected.

WeakMap was introduced in version 8.0.

<?php

    $weakMap = new WeakMap();

    $obj = new stdClass();
    $weakMap[$obj] = 'some data';

    echo $weakMap[$obj]; // some data

    // Now unset $obj:
    unset($obj);

    // The entry is automatically removed from the WeakMap
    var_dump($weakMap); // empty WeakMap

?>

Documentation

See also Hacking PHP’s WeakMap for Value Object D× and WeakMaps a hidden gem in PHP.

Related : Array, [], SplStorageObjects, Garbage Collection, Resource Leak