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首先我们要知道明确单例模式这个概念,那么什么是单例模式呢?

单例模式顾名思义,就是只有一个实例。

作为对象的创建模式, 单例模式确保某一个类只有一个实例,而且自行实例化并向整个系统提供这个实例,

这个类我们称之为单例类。

单例模式的要点有三个:

一是某个类只能有一个实例;

二是它必须自行创建这个实例;

三是它必须自行向整个系统提供这个实例。

下面我们讨论下为什么要使用PHP单例模式?

多数人都是从单例模式的字面上的意思来理解它的用途, 认为这是对系统资源的节省, 可以避免重复实例化, 是一种"计划生育".   而PHP每次执行完页面都是会从内存中清理掉所有的资源. 因而PHP中的单例实际每次运行都是需要重新实例化的, 这样就失去了单例重复实例化的意义了. 单单从这个方面来说, PHP的单例的确有点让各位失望. 但是单例仅仅只有这个功能和应用吗? 答案是否定的,我们一起来看看。

1. php的应用主要在于数据库应用, 所以一个应用中会存在大量的数据库操作, 在使用面向对象的方式开发时(废话), 如果使用单例模式, 则可以避免大量的new 操作消耗的资源。

2. 如果系统中需要有一个类来全局控制某些配置信息, 那么使用单例模式可以很方便的实现. 这个可以参看zend Framework的FrontController部分。

3. 在一次页面请求中, 便于进行调试, 因为所有的代码(例如数据库操作类db)都集中在一个类中, 我们可以在类中设置钩子, 输出日志,从而避免到处var_dump, echo

例子

 1 <?php 2  3 /* 单例模式举例,其要点如下: 4  * 5  * 1. $_instance 必须声明为静态的私有变量 6  * 2. 构造函数和克隆函数必须声明为私有的,这是为了防止外部程序 new 类从而失去单例模式的意义 7  * 3. getInstance()方法必须声明为公有的,必须调用此方法以返回唯一实例的一个引用 8  * 4. ::操作符只能访问静态变量或静态函数 9  *10  */11 12 class People {13     private static $_instance = NULL;14 15     public $height = '';16 17     public $age = '';18 19     private function __construct() {20         $this -> height = '185';21         $this -> age = 25;22     }23 24     private function __clone() {25         //do something26     }27 28     static public function getInstance() {29         if (!self::$_instance instanceof self) {30             //echo 'lgh-big';31             self::$_instance = new self();32         } else {33             //for testing only34             //echo 'gdc-xiaoairener';35         }36 37         return self::$_instance;38     }39 40     public function getHeight() {41         echo $this -> height;42     }43 44     public function getAge() {45         echo $this -> age;46     }47 48 }49 50 function testInstance() {51     People::getInstance() -> getAge();52 }53 54 //begin to use the class55 56 $lgh = People::getInstance();57 $lgh -> getHeight();58 59 echo '<br />';60 testInstance();61 ?>

另特注:
* 5. PHP的单例模式是相对而言的,因为PHP的解释运行机制使得每个PHP页面被解释执行后,所有的相关资源都会被回收。也就是说,PHP在语言级别上没有办法让某个对象常驻内存。在PHP中,所有的变量都是页面级的,无论是全局变量,还是类的静态成员,都会在页面执行完毕后被清空,结果会重新建立新的对象,这样也就完全失去了Singleton的意义。不过,在实际应用中同一个页面中可能会存在多个业务逻辑,这时单例模式就起到了很重要的作用,有效的避免了重复new 对象(注: new 对象会消耗内存资源)这么一个行为,所以我们说PHP的单例模式是相对而言的

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