Can Object Of A Parent Class Access Methods / Properties Of Child Class?

I've began dedicating time to teach myself OOP and PHP and stumbled upon a very basic problem with inheritance. While the answers to the above question is already available online; I thought it'd help many new programmers understand the basics. So here we go.

<?php

class MyClass
{
    public $prop1 = "This is my new property in MyClass <br/>";
   
    public function __construct()
    {
        echo "New Instance Of " . __CLASS__ . "has been created. <br/>";
       
    }
   
    public function __destruct()
    {
        echo "The class" . __CLASS__ . "has been destroyed. <br/>";
    }
   
    public function __toString()
    {
        echo "Now under toString method<br/>";
        return $this-getProperty();
    }
   
    public function setProperty($newVal)
    {
        $this->prop1 = $newVal;
    }
   
    public function getProperty()
    {
        return $this->prop1 . "<br/>";
    }
}

//Extending the MyClass

class MyNewClass extends MyClass
{
   
    //Let's build a new constructor here
    public function __construct()
    {
        echo "I'm a new constructor inside" . __CLASS__ . "I'm inside extended MyClass. <br/>";
    }
   
    //..and one more destructor in here
   
    public function __destruct()
    {
        echo "I'm a destructor of " . __CLASS__ . "inside the extended MyClass. <br/>";
    }
   
   
    public function MyNewMethod()
    {
        echo "Currently Under" . __CLASS__ . "ain't that cool?<br/>";
    }
}

$newobj = new MyNewClass;

echo $newobj->MyNewMethod();

echo "---------<br/>";

$oldobj = new MyClass;

echo $oldobj->; // THIS IS WHERE THE PROBLEM LIES!

?> You see that I'm deliberately trying to echo object so that the toString() method is accessed from MyClass.

The problem:

The constructor and the destructor of MyClass aren't being executed as I'd expect. Would love to know why?