这应该是Singleton的正确方式。

class Singleton {

    private static $instance;
    private $count = 0;

    protected function __construct(){

    }

    public static function singleton(){

        if (!isset(self::$instance)) {

            self::$instance = new Singleton;

        }

        return self::$instance;

    }

    public function increment()
    {
        return $this->count++;
    }

    protected function __clone(){

    }

    protected function __wakeup(){

    }

} 

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。

这应该是Singleton的正确方式。

class Singleton {

    private static $instance;
    private $count = 0;

    protected function __construct(){

    }

    public static function singleton(){

        if (!isset(self::$instance)) {

            self::$instance = new Singleton;

        }

        return self::$instance;

    }

    public function increment()
    {
        return $this->count++;
    }

    protected function __clone(){

    }

    protected function __wakeup(){

    }

} 

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。

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...';
    }
}

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。