In the programming world there are quite a few well understood and explored data structures. Which are commonly used in tons of applications, still the only things PHP offered till 5.3 in regards to structuring data were arrays (more precise: hash tables) and objects. So people had to either abuse them for other structures or implement the structures themselves on top of these. Thanks to Etienne things now are changing and PHP's Standard PHP Library (SPL) extension will offer quite a few standard implementations of data structures. The implemented data structures are implemented using these classes:
Except for SplFixedArray the ideas behind these classes should be clear. If not you can read about them over at Wikipedia or in basically every general introduction to software programming book. For the exact naming and details of the implementation chosen for PHP you can read the fine manual. So why should you use these instead of implementing your own? - There are a few reasons the important ones are:
The only question left there might be: "Ok, that's nice and cool and I could use them, but I have to support older PHP versions for at least some time. What can I do?" - The solution there is easy as well: Most stuff in SPL doesn't do any special trick that requires an implementation in C but can also be implemented in userland. In fact most stuff is tested as userland code first to design the interfaces nicely. Others are implemented in PHP after being implemented in C as kind of documentation. These implementations are distributed with PHP source releases or via CVS, so you can simply take that code and use it.
But now let's get back to the one class which might not be clear: SplFixedArray. SplFixedArray is an implementation of a data structure similar to PHP's arrays with a big limitation: It only supports numeric indexes in a predefined range (starting at 0 and going up) like the ArrayObject from 5.2 it's class based so it can't be used with all thousands of array functions PHP offers, but within it's domain and fucntionality it's very fast and uses less memory than classic PHP arrays. so if you have a big array where numeric indexing is enough: Use it!