<?php

class Base {
    public $alwaysSet = 1;
    public $notAlwaysSet;
    
    public function __construct() {
        $this->notAlwaysSet = 1;
    }
}

class Derived extends Base {
    public function __construct() {
        // do not call parent::__construct()
    }
}

$d = new Derived;
var_dump($d->alwaysSet);
var_dump($d->notAlwaysSet);