PHP5 allows you to register a function (__autoload) which will autosatisfy class dependencies.

Library Code (monkey.php)
<?php
class monkey {
    function scratch() {
        echo "itchy! itchy!\n";
    }
}
?>
Autoload Usage
<?php
function __autoload($classname) {
    include_once("$classname.php");
}

$mono = new monkey;
$mono->scratch();
?>
Output
itchy! itchy!