PHP Singleton design pattern inheritance error

From php singleton class below

<?php
class Singleton
{
    /**
     * @var Singleton The reference to *Singleton* instance of this class
     */
    private static $instance;

    /**
     * Returns the *Singleton* instance of this class.
     *
     * @return Singleton The *Singleton* instance.
     */
    public static function getInstance()
    {
        if (null === static::$instance) {
            static::$instance = new static();
        }

        return static::$instance;
    }

    /**
     * Protected constructor to prevent creating a new instance of the
     * *Singleton* via the `new` operator from outside of this class.
     */
    protected function __construct()
    {
    }
}

I am trying to inherit new child class

class SingletonChild extends Singleton {
}

but when I do testing

$obj = Singleton::getInstance();
$obj_two = SingletonChild::getInstance();
var_dump($obj === Singleton::getInstance());             // bool(true)
var_dump($obj === $obj_two);   // false

I'm getting php fatal error.

PHP Fatal error: Uncaught Error: Cannot access property SingletonChild::$instance