In PHP 5 you can now define the global-scope function __autoload() which can be used to automatically satisify class dependencies at run time

bank_account.class.php
<?php
    class bank_account {

        public $account;

    }
?>
accounts.php
<?php

    function __autoload($classname) {

        include_once("$classname.class.php");

    }

    $a = new bank_account();