$array = [2, 2, 4, 4, 2, 4, 4, 2, 4, 4, 2];
$sum = 8;

$result = [];
foreach($array as $key => $item) {
    foreach($array as $key2 => $item2) {
        if($key != $key2) {
            $calc = $item + $item2;
            if($calc == $sum) {
                array_push($result, [$key, $key2]);
            }
        }

    }
}

$items = new ArrayIterator([2,2,4,2,3,3,6,0,1,3,5,2,2,1]);
$sum = 6;

$getItem = static function($iterator) {
    $result = $iterator->current();
    $iterator->next();
    
    return $result;
};

$result = [];
while($items->valid()) {
    $item = $getItem($items);
    if ($items->valid()) {
        $nextItem = $getItem($items);

        $calc     = $item + $nextItem;
        if ($calc == $sum) {
            array_push($result, [$item, $nextItem]);
        }
    }
}