<?php
//function to load and rename Class1 to Class2: does something like this exist?
load_and_rename_class('Class1', 'Class2');

//now i can extend the renamed class and use the original name:
class Class1 extends Class2{
}
?>

// A class you can't change
class ImmutableClass {
    private function __construct() {
        $this->myObject = new AnotherImmutableClass();
    }
}

$immutable = new ImmutableClass();

// And now you want to call a custom, currently non existing method on myObject
// Because for some reason you need the context that this instance provides
$immutable->myObject->yourCustomMethod();

// Helper function
doSomethingToMyObject($immutable->myObject);
// Or decorator method
$myDecoratedObject = new objectDecorator($immutable->myObject);
$myDecoratedObject->doSomethingToMyObject();