PHP 5.3允许通过后期静态绑定创建可继承的单例类:

class Singleton
{
    protected static $instance = null;

    protected function __construct()
    {
        //Thou shalt not construct that which is unconstructable!
    }

    protected function __clone()
    {
        //Me not like clones! Me smash clones!
    }

    public static function getInstance()
    {
        if (!isset(static::$instance)) {
            static::$instance = new static;
        }
        return static::$instance;
    }
}

这解决了一个问题，在PHP 5.3之前，任何扩展了Singleton的类都会生成父类的实例，而不是它自己的实例。

现在你可以做:

class Foobar extends Singleton {};
$foo = Foobar::getInstance();

$foo将是Foobar的一个实例而不是Singleton的一个实例。

我同意第一个答案，但我也会将类声明为final，这样它就不能被扩展，因为扩展单例违背了单例模式。此外，实例变量应该是私有的，这样就不能直接访问它。还要将__clone方法设为私有，这样就不能克隆单例对象。

下面是一些示例代码。

/**
 * Singleton class
 *
 */
final class UserFactory
{
    private static $_instance = null;

    /**
     * Private constructor
     *
     */
    private function __construct() {}

    /**
     * Private clone method
     *
     */
     private function __clone() {}

    /**
     * Call this method to get singleton
     *
     * @return UserFactory
     */
    public static function getInstance()
    {
        if (self::$_instance === null) {
            self::$_instance = new UserFactory();
        }
        return self::$_instance;
    }
}

示例使用

$user_factory = UserFactory::getInstance();

这阻止你做什么(这将违反单例模式..

你不能这么做!

$user_factory = UserFactory::$_instance;

class SecondUserFactory extends UserFactory { }

制作单例模式的真实和现代方法是:

<?php

/**
 * Singleton Pattern.
 * 
 * Modern implementation.
 */
class Singleton
{
    /**
     * Call this method to get singleton
     */
    public static function instance()
    {
      static $instance = false;
      if( $instance === false )
      {
        // Late static binding (PHP 5.3+)
        $instance = new static();
      }

      return $instance;
    }

    /**
     * Make constructor private, so nobody can call "new Class".
     */
    private function __construct() {}

    /**
     * Make clone magic method private, so nobody can clone instance.
     */
    private function __clone() {}

    /**
     * Make sleep magic method private, so nobody can serialize instance.
     */
    private function __sleep() {}

    /**
     * Make wakeup magic method private, so nobody can unserialize instance.
     */
    private function __wakeup() {}

}

现在你可以用它。

<?php

/**
 * Database.
 *
 * Inherited from Singleton, so it's now got singleton behavior.
 */
class Database extends Singleton {

  protected $label;

  /**
   * Example of that singleton is working correctly.
   */
  public function setLabel($label)
  {
    $this->label = $label;
  }

  public function getLabel()
  {
    return $this->label;
  }

}

// create first instance
$database = Database::instance();
$database->setLabel('Abraham');
echo $database->getLabel() . PHP_EOL;

// now try to create other instance as well
$other_db = Database::instance();
echo $other_db->getLabel() . PHP_EOL; // Abraham

$other_db->setLabel('Priler');
echo $database->getLabel() . PHP_EOL; // Priler
echo $other_db->getLabel() . PHP_EOL; // Priler

正如你所看到的，这种实现更加灵活。

支持多对象，每类一行:

这个方法将在你想要的任何类上强制单例，你所要做的就是向你想要创建单例的类中添加一个方法，这个方法会为你做这件事。

这也将对象存储在“SingleTonBase”类中，因此您可以通过递归SingleTonBase对象来调试您在系统中使用的所有对象。

创建一个名为SingletonBase.php的文件，并将其包含在脚本的根目录中!

代码是

abstract class SingletonBase
{
    private static $storage = array();

    public static function Singleton($class)
    {
        if(in_array($class,self::$storage))
        {
            return self::$storage[$class];
        }
        return self::$storage[$class] = new $class();
    }
    public static function storage()
    {
       return self::$storage;
    }
}

然后，对于任何你想要创建单例的类，只需添加这个小的单方法。

public static function Singleton()
{
    return SingletonBase::Singleton(get_class());
}

这里有一个小例子:

include 'libraries/SingletonBase.resource.php';

class Database
{
    //Add that singleton function.
    public static function Singleton()
    {
        return SingletonBase::Singleton(get_class());
    }

    public function run()
    {
        echo 'running...';
    }
}

$Database = Database::Singleton();

$Database->run();

你可以在任何类中添加这个单例函数，每个类只创建一个实例。

注意:你应该始终将__construct设为private以消除new Class()的使用;实例化。

简单的例子:

final class Singleton
{
    private static $instance = null;

    private function __construct(){}

    private function __clone(){}

    private function __wakeup(){}

    public static function get_instance()
    {
        if ( static::$instance === null ) {
            static::$instance = new static();
        }
        return static::$instance;
    }
}

希望有帮助。

我知道这可能会引起不必要的争论，但是我知道您可能需要多个数据库连接，所以我承认单例可能不是最好的解决方案……然而，我发现单例模式还有其他非常有用的用途。

Here's an example: I decided to roll my own MVC and templating engine because I wanted something really lightweight. However, the data that I want to display contains a lot of special math characters such as ≥ and μ and what have you... The data is stored as the actual UTF-8 character in my database rather than pre-HTML-encoded because my app can deliver other formats such as PDF and CSV in addition to HTML. The appropriate place to format for HTML is inside the template ("view" if you will) that is responsible for rendering that page section (snippet). I want to convert them to their appropriate HTML entities, but PHPs get_html_translation_table() function is not super fast. It makes better sense to retrieve the data one time and store as an array, making it available for all to use. Here's a sample I knocked together to test the speed. Presumably, this would work regardless of whether the other methods you use (after getting the instance) were static or not.

class EncodeHTMLEntities {

    private static $instance = null;//stores the instance of self
    private $r = null;//array of chars elligalbe for replacement

    private function __clone(){
    }//disable cloning, no reason to clone

    private function __construct()
    {
        $allEntities = get_html_translation_table(HTML_ENTITIES, ENT_NOQUOTES);
        $specialEntities = get_html_translation_table(HTML_SPECIALCHARS, ENT_NOQUOTES);
        $this->r = array_diff($allEntities, $specialEntities);
    }

    public static function replace($string)
    {
        if(!(self::$instance instanceof self) ){
            self::$instance = new self();
        }
        return strtr($string, self::$instance->r);
    }
}
//test one million encodings of a string
$start = microtime(true);
for($x=0; $x<1000000; $x++){
    $dump = EncodeHTMLEntities::replace("Reference method for diagnosis of CDAD, but clinical usefulness limited due to extended turnaround time (≥96 hrs)");
}
$end = microtime(true);
echo "Run time: ".($end-$start)." seconds using singleton\n";
//now repeat the same without using singleton
$start = microtime(true);
for($x=0; $x<1000000; $x++){
    $allEntities = get_html_translation_table(HTML_ENTITIES, ENT_NOQUOTES);
    $specialEntities = get_html_translation_table(HTML_SPECIALCHARS, ENT_NOQUOTES);
    $r = array_diff($allEntities, $specialEntities);
    $dump = strtr("Reference method for diagnosis of CDAD, but clinical usefulness limited due to extended turnaround time (≥96 hrs)", $r);
}
$end = microtime(true);
echo "Run time: ".($end-$start)." seconds without using singleton";

基本上，我看到的典型结果如下:

php test.php
Run time: 27.842966794968 seconds using singleton
Run time: 237.78191494942 seconds without using singleton

因此，虽然我不是专家，但我认为没有更方便、更可靠的方法来减少对某些数据的缓慢调用的开销，同时使其超级简单(只需一行代码就可以完成所需的工作)。假设我的例子只有一个有用的方法，因此并不比一个全局定义的函数好，但是一旦你有了两个方法，你就想把它们组合在一起，对吗?我是不是太离谱了?

此外，我更喜欢真正做一些事情的例子，因为有时很难想象一个例子中包含“//在这里做一些有用的事情”这样的语句，这是我在搜索教程时经常看到的。

无论如何，我希望有任何关于为什么在这种情况下使用单例是有害的(或过于复杂)的反馈或评论。