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 behavio

Answer 1

Use trait instead of a abstract class allows to extend a singleton class.

Use the trait SingletonBase for a parent singleton class.

Use the trait SingletonChild for its singleton childs.

interface Singleton
{

    public static function getInstance(): Singleton;

}

trait SingletonBase
{

    private static $instance=null;

    abstract protected function __construct();

    public static function getInstance(): Singleton {

       if (is_null(self::$instance)) {

          self::$instance=new static();

       }

       return self::$instance;

    } 

    protected function clearInstance(): void {

        self::$instance=null;

    }

    public function __clone()/*: void*/ {

        trigger_error('Class singleton '.get_class($this).' cant be cloned.');
    }

    public function __wakeup(): void {

        trigger_error('Classe singleton '.get_class($this).' cant be serialized.');

    }

}

trait SingletonChild
{

    use SingletonBase;

}

class Bar
{

    protected function __construct(){

    }

}

class Foo extends Bar implements Singleton
{

      use SingletonBase;

}

class FooChild extends Foo implements Singleton
{

      use SingletonChild; // necessary! If not, the unique instance of FooChild will be the same as the unique instance of its parent Foo

}