simple service locator + Facade(like in Laravel) implementation

Facade.php

<?php
class Facade{
    private static $service = null;
    public static function getKey(){
        return 'world';
    }
    public static function __callstatic($method,$args){
        if(!isset(self::$service)){
            self::$service = ServiceLocator::getInstance()->getService(self::getKey());
        }
        call_user_func([self::$service,$method],$args);
    }

}

class Hello{
    public function say(){
        echo 'hello';
    }
}
class World{
    public function say(){
        echo 'world ffff';
    }
}

class ServiceLocator{
    private static $instance = null;
    private $container  = null;
    public static  function getInstance(){
        if(!isset(self::$instance)){
            self::$instance = new self;
        }
        return self::$instance;
    }
    private function __construct(){
        $this->container = [
            'hello' => new Hello,
            'world' => new World,
        ];
    }

    public function getService($key){
        if(!isset($this->container[$key])){
            throw new Exception("service $key not exist");
        }
        return $this->container[$key];
    }

}
Facade::say();