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();

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 { }

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

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。

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。

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。