Extending singletons in PHP

Question

I'm working in a web app framework, and part of it consists of a number of services, all implemented as singletons. They all extend a Service class, where the singleton behaviour is implemented, looking something like this:

class Service {     protected static $instance;      public function Service() {         if (isset(self::$instance)) {             throw new Exception('Please use Service::getInstance.');         }     }      public static function &getInstance() {         if (empty(self::$instance)) {             self::$instance = new self();         }         return self::$instance;     } } 

Now, if I have a class called FileService implemented like this:

class FileService extends Service {     // Lots of neat stuff in here } 

... calling FileService::getInstance() will not yield a FileService instance, like I want it to, but a Service instance. I assume the problem here is the "self" keyword used in the Service constructor.

Is there some other way to achieve what I want here? The singleton code is only a few lines, but I'd still like to avoid any code redundance whenever I can.

Answer 1

Code:

abstract class Singleton {     protected function __construct()     {     }      final public static function getInstance()     {         static $instances = array();          $calledClass = get_called_class();          if (!isset($instances[$calledClass]))         {             $instances[$calledClass] = new $calledClass();         }          return $instances[$calledClass];     }      final private function __clone()     {     } }  class FileService extends Singleton {     // Lots of neat stuff in here }  $fs = FileService::getInstance(); 

If you use PHP < 5.3, add this too:

// get_called_class() is only in PHP >= 5.3. if (!function_exists('get_called_class')) {     function get_called_class()     {         $bt = debug_backtrace();         $l = 0;         do         {             $l++;             $lines = file($bt[$l]['file']);             $callerLine = $lines[$bt[$l]['line']-1];             preg_match('/([a-zA-Z0-9\_]+)::'.$bt[$l]['function'].'/', $callerLine, $matches);         } while ($matches[1] === 'parent' && $matches[1]);          return $matches[1];     } } 

Answer 2

Had I paid more attention in 5.3 class, I would have known how to solve this myself. Using the new late static binding feature of PHP 5.3, I believe Coronatus' proposition can be simplified into this:

class Singleton {     protected static $instance;      protected function __construct() { }      final public static function getInstance() {         if (!isset(static::$instance)) {             static::$instance = new static();         }          return static::$instance;     }      final private function __clone() { } } 

I tried it out, and it works like a charm. Pre 5.3 is still a whole different story, though.