单件（Singleton）模式在php5中的实现

原文出处http://en.wikipedia.org/wiki/Singleton_pattern

<?php
class Singleton {
  // object instance
  private static $instance;
  // The private construct prevents instantiating the class externally.  The construct can be
  // empty, or it can contain additional instructions...
  private function __construct() {
    ...
  }
 
  // The clone method prevents external instantiation of copies of the Singleton class,
  // thus eliminating the possibility of duplicate classes.  The clone can be empty, or
  // it can contain additional code, most probably generating error messages in response
  // to attempts to call.
  private function __clone() {
    ...
  }
  //This method must be static, and must return an instance of the object if the object
  //does not already exist.
  public static function getInstance() {
    if (!self::$instance instanceof self) { 
      self::$instance = new self;
    }
    return self::$instance;
  }
  //One or more public methods that grant access to the Singleton object, and its private
  //methods and properties via accessor methods.
  public function doAction() {
    ...
  }
}
 
//usage
Singleton::getInstance()->doAction();
 
?>