On his blog, Dan Scott notes that the PHP language will be getting a native doubly-linked list structure in the coming days (as mentioned by Etienne Kneuss).

This is fabulous news; when I wrote the File_MARC PEAR package, I ended up having to implement a linked list class in PEAR to support it.

Doubly linked lists are normal lists, but the elements also have links to both the previous and next items in the list as well. Two other structures will be added right along with it - SplStack and SplQueue.