PHP SPL是什么

1. 什么是SPL？

SPL是Standard PHP Library（PHP标准库）的缩写。

根据官方定义，它是“a collection of interfaces and classes that are meant to solve standard problems”。但是，目前在使用中，SPL更多地被看作是一种使object（物体）模仿array（数组）行为的interfaces和classes。

2. 什么是Iterator？

SPL的核心概念就是Iterator。这指的是一种Design Pattern，根据《Design Patterns》一书的定义，Iterator的作用是“provide an object which traverses some aggregate structure, abstracting away assumptions about the implementation of that structure.”

wikipedia中说，"an iterator is an object which allows a programmer to traverse through all the elements of a collection, regardless of its specific implementation".……"the iterator pattern is a design pattern in which iterators are used to access the elements of an aggregate object sequentially without exposing its underlying representation".

通俗地说，Iterator能够使许多不同的数据结构，都能有统一的操作界面，比如一个数据库的结果集、同一个目录中的文件集、或者一个文本中每一行构成的集合。

如果按照普通情况，遍历一个MySQL的结果集，程序需要这样写：

// Fetch the "aggregate structure"
$result = mysql_query("SELECT * FROM users");

// Iterate over the structure
while ( $row = mysql_fetch_array($result) ) {
   // do stuff with the row here
}