Description:
------------
see test script, explicit GC does not help

the issues seems present when the given callback to CallbackFilterIterator is bound (eg. not static)

https://3v4l.org/UpmHK

Test script:
---------------
class IteratorWithFilter
{
    public $generator;

    public function __construct(array $data)
    {
        $this->generator = new \ArrayIterator($data);
    }

    public function filter()
    {
        $this->generator = new \CallbackFilterIterator($this->generator, function ($row)  {
            return true;
        });
        
        return $this;
    }
}

function createIterator() {
    $iter = new IteratorWithFilter([['a'], ['b']]);
    return $iter->filter();
}

for ($i = 0; $i < 50 * 1000; $i++) {
    createIterator();
    
    if (($i % 1000) === 0) {
        // gc_collect_cycles(); // explicit GC does not help...
        echo round(memory_get_usage() / (1024 * 1024), 3) . " MiB\n";
    }
}

Expected result:
----------------
reported memory usage should stay below 1 MiB

Actual result:
--------------
0.375 MiB
1.719 MiB
3.071 MiB
4.36 MiB
5.774 MiB
7.064 MiB
8.353 MiB
9.892 MiB
...