SPL提供了一组标准数据结构。

• SplDoublyLinkedList Class：双向链表（DLL）是在两个方向上相互链接的节点列表。当底层结构是dll时，迭代器的操作、对两端的访问、节点的添加或删除均是O(1)，为堆栈和队列提供了良好的实现

  SplDoublyLinkedList::add — Add/insert a new value at the specified index
  SplDoublyLinkedList::bottom — Peeks at the node from the beginning of the doubly linked list
  SplDoublyLinkedList::__construct — Constructs a new doubly linked list
  SplDoublyLinkedList::count — Counts the number of elements in the doubly linked list.
  SplDoublyLinkedList::current — Return current array entry
  SplDoublyLinkedList::getIteratorMode — Returns the mode of iteration
  SplDoublyLinkedList::isEmpty — Checks whether the doubly linked list is empty.
  SplDoublyLinkedList::key — Return current node index
  SplDoublyLinkedList::next — Move to next entry
  SplDoublyLinkedList::offsetExists — Returns whether the requested $index exists
  SplDoublyLinkedList::offsetGet — Returns the value at the specified $index
  SplDoublyLinkedList::offsetSet — Sets the value at the specified $index to $newval
  SplDoublyLinkedList::offsetUnset — Unsets the value at the specified $index
  SplDoublyLinkedList::pop — Pops a node from the end of the doubly linked list
  SplDoublyLinkedList::prev — Move to previous entry
  SplDoublyLinkedList::push — Pushes an element at the end of the doubly linked list
  SplDoublyLinkedList::rewind — Rewind iterator back to the start
  SplDoublyLinkedList::serialize — Serializes the storage
  SplDoublyLinkedList::setIteratorMode — Sets the mode of iteration
  SplDoublyLinkedList::shift — Shifts a node from the beginning of the doubly linked list
  SplDoublyLinkedList::top — Peeks at the node from the end of the doubly linked list
  SplDoublyLinkedList::unserialize — Unserializes the storage
  SplDoublyLinkedList::unshift — Prepends the doubly linked list with an element
  SplDoublyLinkedList::valid — Check whether the doubly linked list contains more nodes

• SplStack Class：该类继承自SplDoublyLinkedList，通过使用一个双向链表来提供栈的主要功能

  SplStack::__construct — Constructs a new stack implemented using a doubly linked list
  SplStack::setIteratorMode — Sets the mode of iteration

• SplQueue Class：该类继承自SplDoublyLinkedList，通过使用一个双向链表来提供队列的主要功能

  SplQueue::__construct — Constructs a new queue implemented using a doubly linked list
  SplQueue::dequeue — Dequeues a node from the queue
  SplQueue::enqueue — Adds an element to the queue.
  SplQueue::setIteratorMode — Sets the mode of iteration

• SplHeap：该abstract类提供了堆的主要功能。堆是满足堆属性的树型结构：每个节点都大于或等于其子节点

  SplHeap::compare — Compare elements in order to place them correctly in the heap while sifting up.
  SplHeap::__construct — Constructs a new empty heap
  SplHeap::count — Counts the number of elements in the heap.
  SplHeap::current — Return current node pointed by the iterator
  SplHeap::extract — Extracts a node from top of the heap and sift up.
  SplHeap::insert — Inserts an element in the heap by sifting it up.
  SplHeap::isEmpty — Checks whether the heap is empty.
  SplHeap::key — Return current node index
  SplHeap::next — Move to the next node
  SplHeap::recoverFromCorruption — Recover from the corrupted state and allow further actions on the heap.
  SplHeap::rewind — Rewind iterator back to the start (no-op)
  SplHeap::top — Peeks at the node from the top of the heap
  SplHeap::valid — Check whether the heap contains more nodes

• SplMaxHeap Class：该类继承自SplHeap，提供了大顶堆的主要功能

  SplMaxHeap::compare — Compare elements in order to place them correctly in the heap while sifting up.

• SplMinHeap class：该类继承自SplHeap，提供了小顶堆的主要功能

  SplMinHeap::compare — Compare elements in order to place them correctly in the heap while sifting up.

• SplPriorityQueue Class：该类提供优先队列的功能，以大顶堆来实现的

  SplPriorityQueue::compare — Compare priorities in order to place elements correctly in the heap while sifting up.
  SplPriorityQueue::__construct — Constructs a new empty queue
  SplPriorityQueue::count — Counts the number of elements in the queue.
  SplPriorityQueue::current — Return current node pointed by the iterator
  SplPriorityQueue::extract — Extracts a node from top of the heap and shift up.
  SplPriorityQueue::insert — Inserts an element in the queue by sifting it up.
  SplPriorityQueue::isEmpty — Checks whether the queue is empty.
  SplPriorityQueue::key — Return current node index
  SplPriorityQueue::next — Move to the next node
  SplPriorityQueue::recoverFromCorruption — Recover from the corrupted state and allow further actions on the queue.
  SplPriorityQueue::rewind — Rewind iterator back to the start (no-op)
  SplPriorityQueue::setExtractFlags — Sets the mode of extraction
  SplPriorityQueue::top — Peeks at the node from the top of the queue
  SplPriorityQueue::valid — Check whether the queue contains more nodes

• SplFixedArray Class：这里的array是以连续方式存储数据的结构，可以通过索引访问，与PHP数组是有序哈希表实现不同；另外，SplFixedArray 拥有固定长度、整数indexes，速度更快

  SplFixedArray::__construct — Constructs a new fixed array
  SplFixedArray::count — Returns the size of the array
  SplFixedArray::current — Return current array entry
  SplFixedArray::fromArray — Import a PHP array in a SplFixedArray instance
  SplFixedArray::getSize — Gets the size of the array
  SplFixedArray::key — Return current array index
  SplFixedArray::next — Move to next entry
  SplFixedArray::offsetExists — Returns whether the requested index exists
  SplFixedArray::offsetGet — Returns the value at the specified index
  SplFixedArray::offsetSet — Sets a new value at a specified index
  SplFixedArray::offsetUnset — Unsets the value at the specified $index
  SplFixedArray::rewind — Rewind iterator back to the start
  SplFixedArray::setSize — Change the size of an array
  SplFixedArray::toArray — Returns a PHP array from the fixed array
  SplFixedArray::valid — Check whether the array contains more elements
  SplFixedArray::__wakeup — Reinitialises the array after being unserialised

• SplObjectStorage Class：该类提供了一个从对象到数据(或忽略数据)的map，是一种对象集合

  SplObjectStorage::addAll — Adds all objects from another storage
  SplObjectStorage::attach — Adds an object in the storage
  SplObjectStorage::contains — Checks if the storage contains a specific object
  SplObjectStorage::count — Returns the number of objects in the storage
  SplObjectStorage::current — Returns the current storage entry
  SplObjectStorage::detach — Removes an object from the storage
  SplObjectStorage::getHash — Calculate a unique identifier for the contained objects
  SplObjectStorage::getInfo — Returns the data associated with the current iterator entry
  SplObjectStorage::key — Returns the index at which the iterator currently is
  SplObjectStorage::next — Move to the next entry
  SplObjectStorage::offsetExists — Checks whether an object exists in the storage
  SplObjectStorage::offsetGet — Returns the data associated with an object
  SplObjectStorage::offsetSet — Associates data to an object in the storage
  SplObjectStorage::offsetUnset — Removes an object from the storage
  SplObjectStorage::removeAll — Removes objects contained in another storage from the current storage
  SplObjectStorage::removeAllExcept — Removes all objects except for those contained in another storage from the current storage
  SplObjectStorage::rewind — Rewind the iterator to the first storage element
  SplObjectStorage::serialize — Serializes the storage
  SplObjectStorage::setInfo — Sets the data associated with the current iterator entry
  SplObjectStorage::unserialize — Unserializes a storage from its string representation
  SplObjectStorage::valid — Returns if the current iterator entry is valid