Stupido problema gerarchia classi -.-"

[KingAntony]

Ok. Ma se lo divido in più files non funziona più.
Esempio:

avvia.php:

<?php
require_once 'content.php';
require_once 'post.php';

$p=new post('Pippo','Pluto');
$p->viewall();
echo $p;

content.php:

<?php 
class content{ 
    protected $_app; 
    protected $_action; 
    protected $_argument; 
     
    function __construct($app, $action, $argument=array()){ 
        $this->_argument=$argument;
    } 
     
    function set($name, $value){ 
        $this->_argument[$name]=$value; 
    } 
     
    function __destruct(){ 
         
    } 
     
    public function __toString() {
        return print_r($this->_argument,true);
    }
}

post.php:

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

Mi da questo errore:
Fatal error: Class 'content' not found in C:\wamp\www\public\test\post.php on line 2

Come risolvo? Grazie.