When unsetting an offset, the element will be removed from the double linked list. So the following code:

<?php

$obj = new SplDoublyLinkedList();

$obj->push(4);
$obj->push(5);
$obj->push(6);

$obj->offsetUnset(1);
echo "Our Linked List:";
print_r($obj);

?>

Will output:

Our Linked List:SplDoublyLinkedList Object
(
    [flags:SplDoublyLinkedList:private] => 0
    [dllist:SplDoublyLinkedList:private] => Array
        (
            [0] => 4
            [1] => 5
            [2] => 6
        )

)
Our New Linked List:SplDoublyLinkedList Object
(
    [flags:SplDoublyLinkedList:private] => 0
    [dllist:SplDoublyLinkedList:private] => Array
        (
            [0] => 4
            [1] => 6
        )

Notice that the element with the index 2 has now the index 1. The original element with index 1 did not only had it's value unset but was also removed from the list.