父类的公共变量在子类中变空？ [重复]

I make another question cuz i think people doesn't understand my old question:

Why a protected variable of parent class come empty?

i.e:

class Father {
protected $body;
function __construct()
{

}
public function run()
{
$this->change();
$this->change2();
}
private function change()
{
$this->body = 'new value';
}
private function change2()
{
$this->body = str_replace('value','........','new value');
}
}

class Child extends Father {
function __construct()
{
echo $this->body;
}
}

$father = new Father();
$father->run();
$child = new Child();

body come empty, i need to continue changing this variable body inside Child class but it come empty, i think the unique solution would be to set body as static.

sry for asking a similar question but i'm trying to understand it.

Edit: if i put all these code of run method inside __construct and in child class call parent::__construct() it will make these changes again and i can't do it again cuz i need to continue changing it in child class.

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I think that OP does not undarstand OOP.

Anyway you can reach this wanted behaviour with using static. protected $body; replace with static protected $body; and $this->body with self::body.