getPrivateParentProperties.php

<?php

class Pappi {
    
    private $prop = "default value";
}


class Child extends Pappi {

    public function setProp(string $value) 
    {
        $this->setValueParentProperty('prop', $value);
    }
    
    public function getProp() : string 
    {
        return $this->getValueParentProperty('prop');
    }
    
    private function getParentProperty(string $prop) : \ReflectionProperty
    {
        $r = new \ReflectionClass(parent::class);
        $p = $r->getProperty($prop);
        $p->setAccessible(true);

        return $p;
    }

    private function getValueParentProperty(string $prop)
    {
        return $this->getParentProperty($prop)
            ->getValue($this);
    }

    private function setValueParentProperty(string $prop, $value) : void
    {
        $res = $this->getParentProperty($prop);
        $res->setValue($this, $value);
    }

}

$c = new Child();
var_dump($c->getProp());
$c->setProp('New value');
var_dump($c->getProp());