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。

我喜欢使用trait的@jose-segura方法，但不喜欢在子类上定义静态变量的需要。下面是一个解决方案，通过将实例缓存在一个静态局部变量中到按类名索引的工厂方法中来避免这种情况:

<?php
trait Singleton {

  # Single point of entry for creating a new instance. For a given
  # class always returns the same instance.
  public static function instance(){
    static $instances = array();
    $class = get_called_class();
    if( !isset($instances[$class]) ) $instances[$class] = new $class();
    return $instances[$class];
  }

  # Kill traditional methods of creating new instances
  protected function __clone() {}
  protected function __construct() {}
}

用法与@jose-segura相同，只是在子类中不需要静态变量。

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 { }

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。

以上答案都是可以的，但我还会再补充一些。

无论谁在2021年来到这里，我都将展示另一个使用单例模式类作为trait的例子，并在任何类中重用它。

<?php

namespace Akash;

trait Singleton
{
    /**
     * Singleton Instance
     *
     * @var Singleton
     */
    private static $instance;

    /**
     * Private Constructor
     *
     * We can't use the constructor to create an instance of the class
     *
     * @return void
     */
    private function __construct()
    {
        // Don't do anything, we don't want to be initialized
    }

    /**
     * Get the singleton instance
     *
     * @return Singleton
     */
    public static function getInstance()
    {
        if (!isset(self::$instance)) {
            self::$instance = new self();
        }

        return self::$instance;
    }

    /**
     * Private clone method to prevent cloning of the instance of the
     * Singleton instance.
     *
     * @return void
     */
    private function __clone()
    {
        // Don't do anything, we don't want to be cloned
    }

    /**
     * Private unserialize method to prevent unserializing of the Singleton
     * instance.
     *
     * @return void
     */
    private function __wakeup()
    {
        // Don't do anything, we don't want to be unserialized
    }
}

所以，像在任何课堂上一样轻松地使用它。假设，我们想在UserSeeder类中实现单例模式。

<?php

class UserSeeder
{
    use Singleton;

    /**
     * Seed Users
     *
     * @return void
     */
    public function seed()
    {
        echo 'Seeding...';
    }
}