//isEmpty is a method of SplDoublyLinkedList 
$obj = new SplDoublyLinkedList();
// Check wither linked list is empty
if ($obj->isEmpty())
{
    $obj->push(2);
    $obj->unshift(10);
}
print_r($obj);
//Output like this
SplDoublyLinkedList Object
(
    [flags:SplDoublyLinkedList:private] => 0
    [dllist:SplDoublyLinkedList:private] => Array
        (
            [0] => 10
            [1] => 2
        )
)