Extend a Singleton with PHP 5.2.X - php

Question

I have this code running on PHP 5.2.6
class Singleton {
static private $instance = false;
private $id = false;
protected function __construct() {
$this->id = uniqid();
}
static public function instance() {
if (!self :: $instance) {
self :: $instance = new self();
}
return self :: $instance;
}
public function get_id() {
return $this->id;
}
}
class Chucknorris extends Singleton {
}
echo "php version = ".phpversion()."<br>";
$singleton = Singleton::instance();
echo get_class($singleton)."<br>";
echo "singleton id = ".$singleton->get_id()."<br>";
$chucknorris = Chucknorris::instance();
echo get_class($chucknorris)."<br>";
echo "chucknorris id = ".$chucknorris->get_id()."<br>";
Here's the output
php version = 5.2.6
Singleton
singleton id = 4ea7dca7d8f23
Singleton
chucknorris id = 4ea7dca7d8f23
When I ask for an instance of Chucknorris, I always get the Singleton one. I'd like to find out a way to extend the Singleton.
I know we can use get_called_class method to do it but it comes only with PHP 5.3. Is there anyway I can extend a Singleton without redefining the design pattern in the extended classes ?

Answer 1

Your best bet in PHP < 5.3 is to use a Singleton Factory:
class Singleton
{
private $id = false;
public function __construct() {
$this->id = uniqid();
}
public function get_id() {
return $this->id;
}
}
class SingletonFactory
{
private static $instance_array = array();
public static function getInstance($class_name)
{
if (!isset(self::$instance_array[$class_name]))
{
self::$instance_array[$class_name] = new $class_name();
}
return self::$instance_array[$class_name];
}
}
class Chucknorris extends Singleton {}
$singleton = SingletonFactory::getInstance('Singleton');
echo get_class($singleton)."<br>";
echo "singleton id = ".$singleton->get_id()."<br>";
$chucknorris = SingletonFactory::getInstance('Chucknorris');
echo get_class($chucknorris)."<br>";
echo "chucknorris id = ".$chucknorris->get_id()."<br>";
The only downside here is that your Singleton constructor is public.. so that's a basic violation of that pattern.
Update:
Here's a version that removes the public constructor (warning: this is getting into messy/hacky/poor design territory)
class Singleton
{
private $id = false;
public function __construct() {
$back = debug_backtrace(false);
if (!isset($back[1]['class']) || $back[1]['class'] != 'SingletonFactory')
{
throw new Exception('Consturctor not available, use SingletonFactory::getInstance("CLASSNAME")');
}
$this->id = uniqid();
}
public function get_id() {
return $this->id;
}
}
class SingletonFactory
{
private static $instance_array = array();
public static function getInstance($class_name)
{
if (!isset(self::$instance_array[$class_name]))
{
self::$instance_array[$class_name] = new $class_name($class_name);
}
return self::$instance_array[$class_name];
}
}
class Chucknorris extends Singleton {}
$singleton = SingletonFactory::getInstance('Singleton');
echo get_class($singleton)."<br>";
echo "singleton id = ".$singleton->get_id()."<br>";
$chucknorris = SingletonFactory::getInstance('Chucknorris');
echo get_class($chucknorris)."<br>";
echo "chucknorris id = ".$chucknorris->get_id()."<br>";
$badchuck = new Chucknorris(); // Exception!

Answer 2

Why don't you simulate the get_class_function if it doesn't exist with 5.3 PHP version ?
This code may answer your question.
if (!function_exists('get_called_class')) {
function get_called_class() {
$bt = debug_backtrace();
$lines = file($bt[1]['file']);
preg_match(
'/([a-zA-Z0-9\_]+)::'.$bt[1]['function'].'/',
$lines[$bt[1]['line']-1],
$matches
);
return $matches[1];
}
}
abstract class Singleton {
private $id = false;
protected function __construct() {
$this->id = uniqid();
}
static public function instance() {
static $instances = array();
$called_class_name = get_called_class();
if (!isset($instances[$called_class_name])) {
$instances[$called_class_name] = new $called_class_name();
}
return $instances[$called_class_name];
}
public function get_id() {
return $this->id;
}
}
class Chucknorris extends Singleton {}
class Brucelee extends Singleton {}
echo "php version = ".phpversion()."<br>";
$chucknorris = Chucknorris::instance();
echo get_class($chucknorris)."<br>";
echo "chucknorris id = ".$chucknorris->get_id()."<br>";
$brucelee = Brucelee::instance();
echo get_class($brucelee)."<br>";
echo "brucelee id = ".$brucelee->get_id()."<br>";

Answer 3

You can redefine just the getinstance method (and the instance itself) in Chucknorris to get an instance of it instead of the parent, but I'm not exactly sure what your end goal is. Just change the extending class to:
class Chucknorris extends Singleton {
static private $instance = false;
static public function instance()
{
if (!self :: $instance) {
self :: $instance = new self();
}
return self :: $instance;
}
}
Is this what you want? And if so - what is the reason you want it? I could think of a few, but would be glad if you share you goal.

Answer 4

Your code will most likely work if you move static private $instance = false; to the subclass and make it protected instead of private.
You also need to replace self:: with static:: so the static var is set in the subclass.
This requires PHP 5.3 - however, this shouldn't be a problem because PHP 5.2 reached end-of-life/support (that includes security updates!) as of january 2011!