autoloading

antes.php

<?php

// config.php
include_once 'a.class.php';
include_once 'b.class.php';
include_once 'c.class.php';
include_once 'd.class.php';

// index.php
include 'config.php';

$a = new A();

depois.php

<?php

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

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