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。

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

    // Prevent cloning and de-serializing
    private function __clone(){}
    private function __wakeup(){}


    /**
     * Call this method to get singleton
     *
     * @return UserFactory
     */
    public static function Instance()
    {
        if ($inst === null) {
            $inst = new UserFactory();
        }
        return $inst;
    }
    
    /**
     * Private ctor so nobody else can instantiate it
     *
     */
    private function __construct()
    {
        
    }
}

使用方法:

$fact = UserFactory::Instance();
$fact2 = UserFactory::Instance();

$fact == $fact2;

But:

$fact = new UserFactory()

抛出错误。

参见http://php.net/manual/en/language.variables.scope.php#language.variables.scope.static了解静态变量的作用域以及为什么设置static $inst = null;的工作原理。

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();

数据库类，检查是否有任何现有的数据库实例，它将返回先前的实例。

   class Database {  
        public static $instance;  
         public static function getInstance(){  
            if(!isset(Database::$instance) ) {  
                Database::$instance = new Database();  
            }  
           return Database::$instance;  
         }  
         private function __cunstruct() {  
           /* private and cant create multiple objects */  
         }  
         public function getQuery(){  
            return "Test Query Data";  
         }  
    }  
    $dbObj = Database::getInstance();  
    $dbObj2 = Database::getInstance();  
    var_dump($dbObj);  
    var_dump($dbObj2);  


/* 
After execution you will get following output: 

object(Database)[1] 
object(Database)[1] 

*/  

Ref http://www.phptechi.com/php-singleton-design-patterns-example.html

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。

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。