lol.php

<?php
class A {
  protected $_member = 'hurr';
  public function get_a() {
    return $this->_member;
  }
}

class B extends A {
  public function set_A_a( $obj, $value ) {
    $obj->_member = $value;
  }
}

$a = new A();
$b = new B();

var_dump($a->get_a());
$b->set_A_a($a, 'im a durr');
var_dump($a->get_a());

output.txt

$ php -f lol.php 
string(4) "hurr"
string(9) "im a durr"