Stupido problema gerarchia classi -.-"

[KingAntony]

Ho incluso le due classi in avvia.php.
Comunque ora funziona ma il problema è questo:
Fatal error: Using $this when not in object context in C:\wamp\www\test\post.php on line 5

avvia.php:

<?php

error_reporting(1);

function __autoload($classname){
    require_once $classname.'.php';
}

$content=new content('post', 'viewall');

content.php:

<?php
class content{
    protected $_app;
    protected $_action;
    protected $_argument;
    
    function __construct($app, $action, $argument=array()){
        $this->_app=$app;
        $this->_action=$action;
        $this->_argument=$argument;
    }
    
    function set($name, $value){
        $this->_argument[$name]=$value;
        echo $name;
    }
    
    function __destruct(){
        if(method_exists($this->_app, $this->_action)){
            extract($this->_argument, EXTR_PREFIX_SAME, "argument");
            call_user_func_array(array($this->_app, $this->_action), $this->_argument);
            require_once ROOT.DS.'app'.DS.$this->_app.DS.$this->_action.'.php';
        }else{
            echo "404";
        }
    }
    
}

post.php:

<?php
class post extends content{ 
    function viewall(){
        $this->set('prova', 'azunyan!'); 
    }          
}

Ho aggiornato il codice.