protected-test.php

<?php
class ParentClass {
    private $descendant;

    public function setDescendant($descendant) {
        $this->descendant = $descendant;
        return $this;
    }

    public function showDescendantValue() {
        echo $this->descendant->value.PHP_EOL;
    }

}

class DescendantClass extends ParentClass {
    protected $value = "DescendantClass value";
}

$parentClass = new ParentClass();
$parentClass
    ->setDescendant(new DescendantClass())
    ->showDescendantValue();