<?php
class parentExample
{
    public function parent(){
        return 'parent';
    }
}

class childrenExample extends parentExample
{

    private $echoecho = '';
    public function __construct($yahooo='fuuuuu!')
    {
        $this->echoecho = $yahooo;
    }

    public function children()
    {
        
        return $this->echoecho . 'children';
    }
}

$parentExample = new parentExample();
var_dump($parentExample->parent());

$childrenExample = new childrenExample('Yahooo!!');
var_dump($childrenExample->children());
var_dump($childrenExample->parent());