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(){

    }

} 

class Database{

        //variable to hold db connection
        private $db;
        //note we used static variable,beacuse an instance cannot be used to refer this
        public static $instance;

        //note constructor is private so that classcannot be instantiated
        private function __construct(){
          //code connect to database  

         }     

         //to prevent loop hole in PHP so that the class cannot be cloned
        private function __clone() {}

        //used static function so that, this can be called from other classes
        public static function getInstance(){

            if( !(self::$instance instanceof self) ){
                self::$instance = new self();           
            }
             return self::$instance;
        }


        public function query($sql){
            //code to run the query
        }

    }


Access the method getInstance using
$db = Singleton::getInstance();
$db->query();

<?php
/**
 * Singleton patter in php
 **/
trait SingletonTrait {
   protected static $inst = null;

  /**
   * call this method to get instance
   **/
   public static function getInstance(){
      if (static::$inst === null){
         static::$inst = new static();
      }
      return static::$inst;
  }

  /**
   * protected to prevent clonning 
   **/
  protected function __clone(){
  }

  /**
   * protected so no one else can instance it 
   **/
  protected function __construct(){
  }
}

使用方法:

/**
 *  example of class definitions using SingletonTrait
 */
class DBFactory {
  /**
   * we are adding the trait here 
   **/
   use SingletonTrait;

  /**
   * This class will have a single db connection as an example
   **/
  protected $db;


 /**
  * as an example we will create a PDO connection
  **/
  protected function __construct(){
    $this->db = 
        new PDO('mysql:dbname=foodb;port=3305;host=127.0.0.1','foouser','foopass');
  }
}
class DBFactoryChild extends DBFactory {
  /**
   * we repeating the inst so that it will differentiate it
   * from UserFactory singleton
   **/
   protected static $inst = null;
}


/**
 * example of instanciating the classes
 */
$uf0 = DBFactoryChild::getInstance();
var_dump($uf0);
$uf1 = DBFactory::getInstance();
var_dump($uf1);
echo $uf0 === $uf1;

respose:

object(DBFactoryChild)#1 (0) {
}
object(DBFactory)#2 (0) {
}

如果你使用的是PHP 5.4: trait，它是一个选项，所以你不必为了拥有单例模式而浪费继承层次结构

还要注意你是使用trait还是扩展Singleton类 如果你不添加以下代码行，就会创建子类的单例:

   protected static $inst = null;

在儿童班

意想不到的结果将是:

object(DBFactoryChild)#1 (0) {
}
object(DBFactoryChild)#1 (0) {
}

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 { }

下面是我的例子，它提供了调用as $var = new Singleton()的能力，并创建了3个变量来测试它是否创建了新对象:

class Singleton{

    private static $data;

    function __construct(){
        if ($this::$data == null){
            $this->makeSingleton();
        }
        echo "<br/>".$this::$data;
    }

    private function makeSingleton(){
        $this::$data = rand(0, 100);
    }

    public function change($new_val){
        $this::$data = $new_val;
    }

    public function printme(){
        echo "<br/>".$this::$data;
    }

}


$a = new Singleton();
$b = new Singleton();
$c = new Singleton();

$a->change(-2);
$a->printme();
$b->printme();

$d = new Singleton();
$d->printme();

我很久以前就写过，想在这里分享

class SingletonDesignPattern {

    //just for demo there will be only one instance
    private static $instanceCount =0;

    //create the private instance variable
    private static $myInstance=null;

    //make constructor private so no one create object using new Keyword
    private function  __construct(){}

    //no one clone the object
    private function  __clone(){}

    //avoid serialazation
    public function __wakeup(){}

    //ony one way to create  object
    public static  function  getInstance(){

        if(self::$myInstance==null){
            self::$myInstance=new SingletonDesignPattern();
            self::$instanceCount++;
        }
        return self::$myInstance;
    }

    public static function getInstanceCount(){
        return self::$instanceCount;
    }

}

//now lets play with singleton design pattern

$instance = SingletonDesignPattern::getInstance();
$instance = SingletonDesignPattern::getInstance();
$instance = SingletonDesignPattern::getInstance();
$instance = SingletonDesignPattern::getInstance();

echo "number of instances: ".SingletonDesignPattern::getInstanceCount();