$a = new SplDoublyLinkedList;
$arr=[1,2,3,4,5,6,7,8,9];

for($i=0;$i<count($arr);$i++){
    $a->add($i,$arr[$i]);
}

print_r($a);

//Output:

SplDoublyLinkedList Object
(
    [flags:SplDoublyLinkedList:private] => 0
    [dllist:SplDoublyLinkedList:private] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
            [3] => 4
            [4] => 5
            [5] => 6
            [6] => 7
            [7] => 8
            [8] => 9
        )

)