WeakMap and SplObjectStorage have different behavior when iterating in a foreach loop. While the key in SplObjectStorage is the numeric index of the inner array, in the WeakMap the key is the object reference:

<?php
class A {}
    
$a = new A;

$objectStorage = new SplObjectStorage ();
$weakMap = new WeakMap();

$objectStorage [$a] = 1;
$weakMap[$a] = 1;

foreach($objectStorage as $key => $value) {
    var_dump($key); // prints: int(0)
}

foreach($weakMap as $key => $value) {
    var_dump($key); // prints: object(A)#1 (0) {}
}