PHP OOP variable scope problem

[Vanilla]

Aight', I've never dug deep into the php OOP thing, as I've always relied on procedural programming or OOP on a very low level...bla...bla...
Silly me,
ANYWAY, I've run into a problem, I just can't solve. I donno' whether I should use $this, parent, self, :: (Scope resolution operator). Or whether the static keywords is even supposed to be there.

The case is, that I have a main class, in which I've instantialized(?) two secondary classes (Main -> One and Main -> Two).
Afterwards, through the main class I've changed instance Two's variable $Xenon to be " in ". But I keep getting an:

Code:

Trying to get property of non-object

- error.

Don't mind the resulting sentence, it's just random stuff
Main.php

PHP Code:

class Main
{
    static public $One, $Two;
    
    function Init()
    {
        require("child.php");
        require("child2.php");
        
        $this->One = new First;
        $this->Two = new Second;
        $this->Two->Xenon = " in ";
        
        $this->One->Activate("the Oven");
    }
}
$Main = new Main;
$Main->Init(); 


child.php

PHP Code:

class First extends Main
{
    function Activate($Arg)
    {
        echo(Second::A().Second::$Xeen.$Two->Xenon.$Arg);
        //Result i want: Mary's pet was burned in the oven
        //Actual result: Mary's pet was burnedthe oven
    }
} 


child2.php

PHP Code:

class Second extends Main
{
    static public $Xeen = " was burned";
    static public $Xenon;
    
    function A()
    {
        return("Mary's Pet");
    }
} 


I've found out, that I can get the result I want if I pinpoint the $Xenon variable from an absolute/global perspective, like this:

PHP Code:

function Activate($Arg)
    {
                      global $Main;

        echo(Second::A().Second::$Xeen.$Main->Two->Xenon.$Arg);
        
    } 


But that kinda sucks, isn't there a relative way to do it? Like where you point to the parent and then point to subject from there? Hope you understand what I mean.

Regards and thanks in advance
- Vanilla