Note: I’ve written on this topic before, but thought the subject warranted further more detailed discussion and a more comprehensive and up-to-date set of benchmarks. Hence, this post and this presentation. Enjoy.
The SPL, or Standard PHP Library, is an often overlooked extension in the PHP core. It first came on the scene in PHP 5 and a variety of iterators constituted the majority of its initial offerings. Though the iterator offerings were expanded in PHP 5.3, the particularly interesting additions to the SPL were several specialized data structure classes, the foundational concepts for which originate in the field of computer science. In this post, I will provide an overview of these new classes and explain why and when they should be used.
While PHP has several data types, the ones that likely see the most frequent and varied use are strings and arrays. They are the proverbial duct tape and WD-40 of PHP, respectively. Like arrays, SPL data structure classes are used to store composite (i.e. non-scalar) data.
Now, that’s not to say that every instance of an array in existing codebases should be replaced with an SPL container object. There are cases where it’s appropriate to use one over the other. Knowing the difference requires an understanding of how arrays work.
Within the C code that makes up the PHP interpreter, arrays are implemented as a data structure called a hash table or hash map. When a value contained within an array is referenced by its index, PHP uses a hashing function to convert that index into a unique hash representing the location of the corresponding value within the array.
This hash map implementation enables arrays to store an arbitrary number of elements and provide access to all of those elements simultaneously using either numeric or string keys. Arrays are extremely fast for the capabilities they provide and are an excellent general purpose data structure.
In contrast to arrays,
SplFixedArray functions more like C arrays or Java arrays than PHP arrays. The maximum number of elements that it may contain is specified upon instantiation. While it is possible to change it later via the
setSize() method, this negates the performance advantages of using it: because its size is fixed, it doesn’t need to use a hashing function to resolve the position of elements within the array. It makes sense to use fixed arrays when the number of elements to be stored is known in advance and the elements only need to be accessed by sequential position.
SplFixedArray implements the
Iterator allows it to be iterated using a
ArrayAccess provides access to its elements using array syntax where elements are referred to using integer positions beginning at 0 as with enumerated arrays.
Countable enables a list to be passed to the
count() function like an array.
Aside from the inability to use it in place of arrays with array functions, instances of
SplFixedArray function just like arrays for all intents and purposes. It’s even possible to convert them to and from arrays using the
fromArray() methods respectively. However, it generally makes more sense to use
SplFixedArray exclusively for each individual use case.
In computer science, a list is defined as an ordered collection of values. A linked list is a data structure in which each element in the list includes a reference to one or both of the elements on either side of it within the list. The term “doubly-linked list” is used to refer to the latter case. In the SPL, this takes the form of the class
SplDoublyLinkedList also implements the
Countable interfaces. In addition to the methods that come with these interface implementations, elements can be added to or removed from the start or end of the list using its
unshift() methods, which correspond to the
array_unshift() functions respectively. Unfortunately, as of PHP 5.3.4, there’s no way to insert an element anywhere in the list other than at the beginning or the end. A feature request has been filed for this. Add a comment or vote to show support for its addition.
The elements at the start and end of the list are accessible via its
bottom() methods respectively, which correspond to the
end() functions. Like
SplFixedArray, elements can also be accessed arbitrarily by positional index using the array syntax granted by
ArrayAccess. It makes sense to use lists when the number of elements to be stored is not known in advance and the elements only need to be accessed by sequential position.
Stacks are similar to lists with two major differences. First, elements can only be added to the top of the stack. Second, an element can only be accessed by taking it off the top of the stack. Because of these differences, the stack is often referred to as a Last-In-First-Out or LIFO data structure.
SplStack is the SPL stack implementation.
SplStack is a bit removed from the traditional definition of a stack. It extends
SplDoublyLinkedList and inherits its abilities, some of which don’t really apply to stacks. In order to enforce its restriction on how elements are accessed,
SplStack overrides the
setIteratorMode() method of its parent class and implements its own to prevent modification of the iteration direction. Both methods allow elements to be retained or removed as they are iterated.
Use of stacks makes sense when the number of elements to be stored is not known in advance and the only element that must be accessible is the last one stored. However, as of PHP 5.3.4, the performance of
SplStack leaves something to be desired. Benchmarks included later in this provide an objective illustration of this, though the cause of the behavior remains unknown.
Queues are also similar to lists, again with two major differences. First, elements can only be added (or “enqueued”) to the end of the queue. Second, an element can only be accessed by removing (or “dequeueing”) it from the beginning of the queue. For these reasons the queue is referred to as a First-In-First-Out or FIFO data structure. The
SplQueue class implements this data structure in the SPL.
SplQueue follows suit with
SplStack in extending
SplDoublyLinkedList. Just as
SplStack resultingly inherits some operations with at least questionable applicability, so too does
SplQueue. Likewise, it overrides
setIteratorMode() with its own version to restrict how elements are accessed. Use of queues makes sense when the number of elements to be stored is not known in advance and the only element that must be accessible is the remaining element that was stored earliest.
One minor difference between
SplStack is that the former contains two method aliases named after conceptual queue operations:
SplDoublyLinkedList::push(). This makes sense because while
pop() share similar applicability to conceptual stack operations, they are already present in its parent class.
Despite their common ancestry,
SplQueue appears to have better performance than
SplStack as of PHP 5.3.4. Benchmarks included later in this post review this in more detail.
Up to this point, the data structures discussed have resembled lists insofar as they contain elements in the order in which they were added. By contrast, when an element is added to a heap, a comparison function is used to compare the new element to other elements already in the heap and element is placed appropriately within the heap based on that function’s return value. The beauty of heaps is that their underlying algorithm does this with minimal element comparisons, so it’s extremely efficient. Using heaps makes sense when the number of elements to be stored is not known in advance and elements must be accessed in an order based on how they compare to each other.
SplHeap is an abstract class used to create a heap by extending it and providing a comparison function in the form of its
compare() method. Only the root element of a heap, the one yielding the highest comparison function return value, may be accessed or removed from the heap at any given time. This is done using the
extract() method of
SplHeap implements the
Countable interfaces but, because only the root element can be extracted, it does not implement the
ArrayAccess interface like the previously discussed data structure classes.
In addition to the abstract
SplHeap class, two concrete implementations are also included in the SPL, namely
compare() method of
SplMinHeap returns a value such that the smallest element in the heap is the root element. Likewise, the
compare() method of
SplMaxHeap returns a value such that the largest element in the heap is the root element.
At first glance, using a subclass of
SplHeap may seem equivalent to calling
sort() or a similar function on an array and accessing the elements in sequence. This is indeed the case if all elements are added to the array prior to it being sorted. However, situations such as elements arriving over time or inadequate memory to store all elements simultaneously may preclude this approach. Use of arrays in such situations would require repeated resorting of the entire array as new elements are added, which is inefficient. This is why using the corresponding heap class makes a lot more sense in that situation than repeated calls to
SplHeap can be used to implement the heapsort algorithm, which has better worst case performance than the quicksort algorithm implementation used by arrays.
Priority queues are somewhat similar to heaps. In fact, while it doesn’t extend
SplPriorityQueue does make use of a heap structure internally to implement its functionality. The difference is that the
insert() method of
SplPriorityQueue queue accepts both a value and an associated priority, removing the need to use an array or object to store both of these and define an appropriate comparison function in an
SplHeap instance. Elements with the highest priority, like those in
SplMaxHeap with the highest value, are the ones that come out first when
extract() is called. Note that elements with equal priority are returned in no particular order.
For reasons similar to those of
SplPriorityQueue implements both
Countable interfaces and does not implement the
ArrayAccess interface. Because it stores a value and priority per element,
SplPriorityQueue includes a
setExtractFlags() method that modifies the behavior of
extract() to return the stored value, the stored priority, or an array containing both. Priorities are not bound to a particular data type: strings, integers, or even composite data types can be used.
SplPriorityQueue can be extended and its
compare() method overridden to customize the comparison logic.
It makes sense to use a priority queue when the number of elements to be stored is not known in advance and elements must be accessed in an order based on how a value associated with each element (versus the element value itself) compares to the same associated values of other elements.
Sets and Composite Hash Maps
SplObjectStorage combines some of the properties of two different data structures. First, it provides the same functionality of a hash table that a normal array has, but without its associated inability to use objects as keys unless the
spl_object_hash() function is used. In other words, it implements a composite hash map. Second, it can be used as a set to store objects as data without a meaningful corresponding key or concept of sequential order.
attach() method accepts an object key and the data to associate with it and its
detach() method allows data to be removed using its associated object key. To use the object as a set, simply exclude the
$data parameter for
attach() as it’s optional. The set operations implemented by
SplObjectStorage all have array function counterparts. For example, the
addAll() method and
array_merge() function both correspond to the union set operation. The difference operation is available using the
removeAll() method and
array_diff() function and its variants. The
contains() method and
in_array() function both implement the element_of operation. Sadly, only arrays have an implementation of the intersection operation in the form of
array_intersect() and its variants. Tobias Schlitt has a more in-depth analysis of this data structure that includes implementations of the set operations lacking in the SPL itself.
Like some of the other data structures in the SPL,
SplObjectStorage implements the
ArrayAccess interfaces. Oddly, it also implements the
Traversable interface (which is limited to internally defined classes and negates the need for implementation of the
Iterator interface) and the
Serializable interface (and it is the only SPL data structure class to do so).
Using this class makes sense when data must be stored using composite keys or the ability to access data using set operations is more important than accessing data in a specific order.
Code and instructions are located in this GitHub repository.
Update: I’ve removed benchmark result charts generated on my system from this post. I encourage you to run the benchmarks yourself to get your own results. (4/14/2015)
Other Data Structures
If you have an interest in other data structure implementations for PHP outside of SPL offerings, check out the bloomy PECL extension, which is an implementation of a bloom filter created by Andrei Zmievski.