PHP Inheritance Example. https://www.reddit.com/r/PHPhelp/comments/5rro8m/help_with_inheritance_and_protectedprivate/

inheritance.php

<?php

class Supervisor {
    private $prop1;
    private $prop2;
    protected $key;

    protected function set($key, $val) {
        $this->$key = $val;
    }

    protected function get($key) {
        return $this->$key;
    }
}

class Child extends Supervisor {
    private $childProp1;
    private $childProp2;

    public function doStuff() {
        // work goes here
    }

    public function set($key, $val) {
        parent::set($key, $val);
    }

    public function get($key) {
        return parent::get($key);
    }
}


$slave = new Child();
$slave->set('asdf', 'Timmy');

echo $slave->get('asdf') . PHP_EOL; // Timmy gets written out.