WeakMap::offsetExists return false if value is null

Description

The following code:

<?php

$wm = new \WeakMap;
$os = new \SplObjectStorage;

$obj1 = new \stdClass;
$obj2 = new \stdClass;

$wm[$obj1] = null;
$wm[$obj2] = true;

$os[$obj1] = null;
$os[$obj2] = true;


var_dump($wm->offsetExists($obj1));
var_dump($wm->offsetExists($obj2));

var_dump($os->offsetExists($obj1));
var_dump($os->offsetExists($obj2));


foreach ($wm as $key => $value) {
    if ($key === $obj1) {
        echo 'key $obj1 is always here';
    }
}

Resulted in this output:

bool(false)
bool(true)
bool(true)
bool(true)
key $obj1 is always here

But I expected this output instead:

bool(true)
bool(true)
bool(true)
bool(true)
key $obj1 is always here

PHP Version

PHP 8.1.5

Operating System